xfs: rename diff_two_keys routines

One may think that diff_two_keys routines are used to compute the actual
difference between the arguments but they return a result of a
three-way-comparison of the passed operands. So it looks more appropriate
to denote them as cmp_two_keys.

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>