xfs: rename key_diff routines

key_diff routines compare a key value with a cursor value. Make the naming
to be a bit more self-descriptive.

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>