xfs: Use abs_diff instead of XFS_ABSDIFF

We have a central definition for this function since 2023, used by
a number of different parts of the kernel.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>