scsi: target: tcmu: wait for nl reply only if there are listeners or during an add

genlmsg_multicast_allns now returns the correct statuses when a message is
sent to a listener. However in the case of adding a device we want to wait
for the listener otherwise we may miss the the device during startup.

Signed-off-by: Cathy Avery <cavery@redhat.com>
Acked-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>