tcm_loop: Send I_T_NEXUS_LOSS_OCCURRED UA

If the virtual SAS link is set to 'offline' we should be
queueing an I_T_NEXUS_LOSS_OCCURRED UA.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>