f2fs: correctly fix the parent inode number during fsync()

fsync() may be called on a deleted file that's still open.  So when
fsync() tries to set the parent inode number when the inode has
LOST_PINO and i_nlink == 1 (to avoid later checkpoints), it needs to
make sure to get the parent directory via a non-deleted alias.

Also remove the unnecessary igrab() and iput(), as the caller already
holds a reference to the inode.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>