ikev2: Ignore IKEV2_MESSAGE_ID_SYNC notifies if extension is disabled

ikev2: Ignore IKEV2_MESSAGE_ID_SYNC notifies if extension is disabled

If this is the first message by the peer, i.e. we expect MID 0, the
message is not pre-processed in the task manager so we ignore it in the
task.

We also make sure to ignore such messages if the extension is disabled
and the peer already sent us one INFORMATIONAL, e.g. a DPD (we'd otherwise
consider the message with MID 0 as a retransmit).