scsi: target: use the stack for XCOPY passthrough cmds

Reads and writes in the XCOPY loop are synchronous, so needn't be heap
allocated / freed with each loop.

Link: https://lore.kernel.org/r/20200327141954.955-6-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>