scsi: sg: update comment for blk_get_request()

The calling convention of blk_get_request() has changed in lk 4.18; update
the comment in sg.c to match.

Fixes: ff005a0 ("block: sanitize blk_get_request calling conventions")
Signed-off-by: Tony Battersby <tonyb@cybernetics.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/sg.c

@@ -1741,15 +1741,11 @@ sg_start_req(Sg_request *srp, unsigned char *cmd)
 	 *
 	 * 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);