MINOR: wdt: use is_sched_alive() instead of keeping a local ctxsw copy

MINOR: wdt: use is_sched_alive() instead of keeping a local ctxsw copy

Now we can simply call is_sched_alive() on the local thread to verify
that the scheduler is still ticking instead of having to keep a copy of
the ctxsw and comparing it. It's cleaner, doesn't require to maintain
a local copy, doesn't rely on activity[] (whose purpose is mainly for
observation and debugging), and shows how this could be extended later
to cover other use cases. Practically speaking this doesn't change
anything however, the algorithm is still the same.