scsi: scsi_dh_alua: fix missing kref_put() in alua_rtpg_work()

Reference count of pg leaks in alua_rtpg_work() since kref_put() is not
called to decrease the reference count of pg when the condition
pg->rtpg_sdev==NULL satisfied (actually it is easy to satisfy), it would
cause memory of pg leakage.

Signed-off-by: tang.junhui <tang.junhui@zte.com.cn>
Cc: <stable@vger.kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>