scsi: sg: Fix type of last blk_trace_setup() argument

Avoid that sparse reports the following:

drivers/scsi/sg.c:1114:41: warning: incorrect type in argument 5 (different address spaces)
drivers/scsi/sg.c:1114:41:    expected char [noderef] <asn:1>*arg
drivers/scsi/sg.c:1114:41:    got char *<noident>

This patch does not change any functionality.
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/sg.c

@@ -1110,8 +1110,7 @@ sg_ioctl(struct file *filp, unsigned int cmd_in, unsigned long arg)
 		return blk_trace_setup(sdp->device->request_queue,
 				       sdp->disk->disk_name,
 				       MKDEV(SCSI_GENERIC_MAJOR, sdp->index),
-				       NULL,
-				       (char *)arg);
+				       NULL, p);
 	case BLKTRACESTART:
 		return blk_trace_startstop(sdp->device->request_queue, 1);
 	case BLKTRACESTOP: