libxfs: cosmetic changes to libxfs_inode_alloc

libxfs: cosmetic changes to libxfs_inode_alloc

This pre-patch helps make the next libxfs-sync for 5.11 a bit
more clear.

In reality, the libxfs_inode_alloc function matches the kernel's
xfs_dir_ialloc so rename it for clarity before the rest of the
sync, and change several variable names for the same reason.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>