This patch rebuild sit page from sit info in mem instead
of issue a read io.

I test this method and the result is as below:

Pre:
 mmc_perf_test-12061 [001] ...1   976.819992: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = start flush sit
 mmc_perf_test-12061 [001] ...1   976.856446: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = end flush sit
 mmc_perf_test-12061 [003] ...1   998.976946: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = start flush sit
 mmc_perf_test-12061 [003] ...1   999.023269: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = end flush sit
 mmc_perf_test-12061 [003] ...1  1022.060772: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = start flush sit
 mmc_perf_test-12061 [003] ...1  1022.111034: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = end flush sit
 mmc_perf_test-12061 [002] ...1  1070.127643: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = start flush sit
 mmc_perf_test-12061 [003] ...1  1070.187352: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = end flush sit
 mmc_perf_test-12061 [003] ...1  1095.942124: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = start flush sit
 mmc_perf_test-12061 [003] ...1  1095.995975: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = end flush sit
 mmc_perf_test-12061 [003] ...1  1122.535091: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = start flush sit
 mmc_perf_test-12061 [003] ...1  1122.586521: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = end flush sit
 mmc_perf_test-12061 [001] ...1  1147.897487: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = start flush sit
 mmc_perf_test-12061 [001] ...1  1147.959438: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = end flush sit
 mmc_perf_test-12061 [003] ...1  1177.926951: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = start flush sit
 mmc_perf_test-12061 [002] ...1  1177.976823: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = end flush sit
 mmc_perf_test-12061 [002] ...1  1204.176087: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = start flush sit
 mmc_perf_test-12061 [002] ...1  1204.239046: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = end flush sit

Some sit flush consume more than 50ms.

Now:
mmc_perf_test-2187  [007] ...1   196.840684: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = start flush sit
mmc_perf_test-2187  [007] ...1   196.841258: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = end flush sit
mmc_perf_test-2187  [007] ...1   219.430582: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = start flush sit
mmc_perf_test-2187  [007] ...1   219.431144: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = end flush sit
mmc_perf_test-2187  [002] ...1   243.638678: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = start flush sit
mmc_perf_test-2187  [000] ...1   243.638980: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = end flush sit
mmc_perf_test-2187  [002] ...1   265.392180: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = start flush sit
mmc_perf_test-2187  [002] ...1   265.392245: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = end flush sit
mmc_perf_test-2187  [000] ...1   290.309051: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = start flush sit
mmc_perf_test-2187  [000] ...1   290.309116: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = end flush sit
mmc_perf_test-2187  [003] ...1   317.144209: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = start flush sit
mmc_perf_test-2187  [003] ...1   317.145913: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = end flush sit
mmc_perf_test-2187  [005] ...1   343.224954: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = start flush sit
mmc_perf_test-2187  [005] ...1   343.225574: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = end flush sit
mmc_perf_test-2187  [000] ...1   370.239846: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = start flush sit
mmc_perf_test-2187  [000] ...1   370.241138: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = end flush sit
mmc_perf_test-2187  [001] ...1   397.029043: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = start flush sit
mmc_perf_test-2187  [001] ...1   397.030750: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = end flush sit
mmc_perf_test-2187  [003] ...1   425.386377: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = start flush sit
mmc_perf_test-2187  [003] ...1   425.387735: f2fs_write_checkpoint: dev = (259,44), checkpoint for Sync, state = end flush sit

Most sit flush consume no more than 1ms.

Signed-off-by: Yunlei He <heyunlei@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>