target/user: Use sense_reason_t in tcmu_queue_cmd_ring

Instead of using -ERROR-style returns, use sense_reason_t. This lets us
remove tcmu_pass_op(), and return more correct sense values.

Signed-off-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Bryant G. Ly <bryantly@linux.vnet.ibm.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>