scsi: target: increase XCOPY I/O size

The I/O size is already bound by dev_attrib.hw_max_sectors, so increase the
hardcoded XCOPY_MAX_SECTORS maximum to improve performance against
backstores with high-latency.

Link: https://lore.kernel.org/r/20200327141954.955-5-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>