Clear knotes when finishing wseventvar

Clear knotes when finishing wseventvar

When finishing a wseventvar in wsevent_fini(), clear the klist.
Otherwise knotes can be left dangling, which can crash the kernel.

In general, klist_invalidate() should happen after vdevgone() in order
to avoid a race with kevent registration. However, the current wscons
drivers clear the wsevent pointer (sc->sc_base.me_evp) before calling
wsevent_fini(). This prevents the drivers from registering new kevents.

Prompted by a report by Peter J. Philipp on bugs@

OK mvs@ miod@