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>