xfs: refactor xfs_btree_diff_two_ptrs() to take advantage of cmp_int()

Use cmp_int() to yield the result of a three-way-comparison instead of
performing subtractions with extra casts. Thus also rename the function
to make its name clearer in purpose.

Found by Linux Verification Center (linuxtesting.org).

Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>