nvmet-loop: use blk_rq_payload_bytes for sgl selection

blk_rq_bytes does the wrong thing for special payloads like discards and
might cause the driver to not set up a SGL.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Keith Busch <keith.busch@intel.com>