When finsert is doing with dirting pages, we should increase i_size right away.
Otherwise, the moved page is able to be dropped by the following
filemap_write_and_wait_range before updating i_size.
Especially, it can be done by
	if ((page->index >= end_index + 1) || !offset)
		goto out;
in f2fs_write_data_page.

This should resolve the below xfstests/091 failure reported by Dave.

$ diff -u tests/generic/091.out /home/dave/src/xfstests-dev/results//f2fs/generic/091.out.bad
--- tests/generic/091.out       2014-01-20 16:57:33.000000000 +1100
+++ /home/dave/src/xfstests-dev/results//f2fs/generic/091.out.bad       2016-02-08 15:21:02.701375087 +1100
@@ -1,7 +1,18 @@
 QA output created by 091
 fsx -N 10000 -l 500000 -r PSIZE -t BSIZE -w BSIZE -Z -R -W
-fsx -N 10000 -o 8192 -l 500000 -r PSIZE -t BSIZE -w BSIZE -Z -R -W
-fsx -N 10000 -o 32768 -l 500000 -r PSIZE -t BSIZE -w BSIZE -Z -R -W
-fsx -N 10000 -o 8192 -l 500000 -r PSIZE -t BSIZE -w BSIZE -Z -R -W
-fsx -N 10000 -o 32768 -l 500000 -r PSIZE -t BSIZE -w BSIZE -Z -R -W
-fsx -N 10000 -o 128000 -l 500000 -r PSIZE -t BSIZE -w BSIZE -Z -W
+mapped writes DISABLED
+skipping insert range behind EOF
+skipping insert range behind EOF
+truncating to largest ever: 0x11e00
+dowrite: write: Invalid argument
+LOG DUMP (7 total operations):
+1(  1 mod 256): SKIPPED (no operation)
+2(  2 mod 256): SKIPPED (no operation)
+3(  3 mod 256): FALLOC   0x2e0f2 thru 0x3134a  (0x3258 bytes) PAST_EOF
+4(  4 mod 256): SKIPPED (no operation)
+5(  5 mod 256): SKIPPED (no operation)
+6(  6 mod 256): TRUNCATE UP    from 0x0 to 0x11e00
+7(  7 mod 256): WRITE    0x73400 thru 0x79fff  (0x6c00 bytes) HOLE
+Log of operations saved to "/mnt/test/junk.fsxops"; replay with --replay-ops
+Correct content saved for comparison
+(maybe hexdump "/mnt/test/junk" vs "/mnt/test/junk.fsxgood")

Reported-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>