The target sometimes sends a logout request to libiscsi
in case it is going down or for some other reason.
The opcode of such a request is ISCSI_PDU_ASYNC_MSG.
On receiving these kinds of PDU, there is no related pdu on the
list of iscsi->waitpdu. Just skip finding them from iscsi->waitpdu.
Or segment fault might happen.
Also rename nop_target label to no_waitpdu to be more clear.
Signed-off-by: Hou Pu <houpu@bytedance.com>