xfs: Fix comments mentioning xfs_ialloc

Since kernel commit 1abcf261 ("xfs: move on-disk inode allocation out of xfs_ialloc()"),
xfs_ialloc has been renamed to xfs_init_new_inode. So update this in comments.

Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>