nvme: Set integrity flag for user passthrough commands

If the command a separate metadata buffer attached, the request needs
to have the integrity flag set so the driver knows to map it.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>