scsi: target: avoid per-loop XCOPY buffer allocations

The main target_xcopy_do_work() loop unnecessarily allocates an I/O buffer
with each synchronous READ / WRITE pair. This commit significantly reduces
allocations by reusing the XCOPY I/O buffer when possible.

Link: https://lore.kernel.org/r/20200327141954.955-4-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>