scsi: target_core_user: fix double unlock

The caller of queue_cmd_ring grabs and releases the lock, so the
tcmu_setup_cmd_timer failure handling inside queue_cmd_ring should not call
mutex_unlock.

Signed-off-by: Mike Christie <mchristi@redhat.com>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>