scsi: target: use #define for xcopy descriptor len

Link: https://lore.kernel.org/r/20200327141954.955-2-ddiss@suse.de


Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>