tcmu: prep queue_cmd_ring to be used by unmap wq

In the next patches we will call queue_cmd_ring from the submitting
context and also the completion path. This changes the queue_cmd_ring
return code so in the next patches we can return a sense_reason_t
and also signal if a command was requeued.

Signed-off-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>