ikev2: Properly keep track of pending MOBIKE updates

ikev2: Properly keep track of pending MOBIKE updates

Because we only queue one MOBIKE task at a time, but destroy superfluous
ones only after we already increased the counter for pending MOBIKE updates,
we have to reduce the counter when such tasks are destroyed.  Otherwise, the
queued task would assume another task is queued when it is running and
ignore any successful response.