scsi: sg: update comment for blk_get_request()

	 *

	 * With scsi-mq enabled, there are a fixed number of preallocated

	 * requests equal in number to shost->can_queue.  If all of the

	 * preallocated requests are already in use, then using GFP_ATOMIC with

	 * blk_get_request() will return -EWOULDBLOCK, whereas using GFP_KERNEL

	 * will cause blk_get_request() to sleep until an active command

	 * completes, freeing up a request.  Neither option is ideal, but

	 * GFP_KERNEL is the better choice to prevent userspace from getting an

	 * unexpected EWOULDBLOCK.

	 *

	 * With scsi-mq disabled, blk_get_request() with GFP_KERNEL usually

	 * does not sleep except under memory pressure.

	 * preallocated requests are already in use, then blk_get_request()

	 * will sleep until an active command completes, freeing up a request.

	 * Although waiting in an asynchronous interface is less than ideal, we

	 * do not want to use BLK_MQ_REQ_NOWAIT here because userspace might

	 * not expect an EWOULDBLOCK from this condition.

	 */

	rq = blk_get_request(q, hp->dxfer_direction == SG_DXFER_TO_DEV ?

			REQ_OP_SCSI_OUT : REQ_OP_SCSI_IN, 0);