xfs: use a proper variable name and type for storing a comparison result

Perhaps that's just my silly imagination but 'diff' doesn't look good for
the name of a variable to hold a result of a three-way-comparison
(-1, 0, 1) which is what ->cmp_key_with_cur() does. It implies to contain
an actual difference between the two integer variables but that's not true
anymore after recent refactoring.

Declaring it as int64_t is also misleading now. Plain integer type is
more than enough.

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>