target/user: Fix UFLAG_UNKNOWN_OP handling

target/user: Fix UFLAG_UNKNOWN_OP handling

Calling transport_generic_request_failure() from here causes list
corruption. We should be using target_complete_cmd() instead.

Which we do in all other cases, so the UNKNOWN_OP case can become just
another member of the big else/if chain in tcmu_handle_completion().

Signed-off-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
Orabug: 25395066
(cherry picked from commit ed97d0cd78a337450e17eb613bdeec15e729af46)
Signed-off-by: Kyle Fortin <kyle.fortin@oracle.com>
Reviewed-by: Shan Hai <shan.hai@oracle.com>