Previously, f2fs didn't track the parent inode number correctly which is stored
in each f2fs_inode. In the case of the following scenario, a bug can be occured.

Let's suppose there are one directory, "/b", and two files, "/a" and "/b/a".
 - pino of "/a" is ROOT_INO.
 - pino of "/b/a" is DIR_B_INO.

Then,
 # sync
  : The inode pages of "/a" and "/b/a" contain the parent inode numbers as
    ROOT_INO and DIR_B_INO respectively.
 # mv /a /b/a
  : The parent inode number of "/a" should be changed to DIR_B_INO, but f2fs
    didn't do that. Ref. f2fs_set_link().

In order to fix this clearly, I added i_pino in f2fs_inode_info, and whenever
it needs to be changed like in f2fs_add_link() and f2fs_set_link(), it is
updated temporarily in f2fs_inode_info.

And later, f2fs_write_inode() stores the latest information to the inode pages.
For power-off-recovery, f2fs_sync_file() triggers simply f2fs_write_inode().

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>