scsi: target: Split out COMPARE AND WRITE memcmp into helper

In preparation for finding and returning the miscompare offset.

Link: https://lore.kernel.org/r/20201031233211.5207-4-ddiss@suse.de


Reviewed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>