This patch tries to avoid broken symlink case after POR in best effort.
This results in performance regression.
But, if f2fs has inline_data and the target path is under 3KB-sized long,
the page would be stored in its inode_block, so that there would be no
performance regression.

Note that, if user wants to keep this file atomically, it needs to trigger
dir->fsync.
And, there is still a hole to produce broken symlink.
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>