scsi: scsi_dh_alua: Warn if the first argument of alua_rtpg_queue() is NULL

Callers must provide a valid port group to alua_rtpg_queue().  Issue a
kernel warning if that is not the case.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Tang Junhui <tang.junhui@zte.com.cn>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>