Avoid unsynchronized updates to kn_status.

kn_status is protected by the kqueue's lock, but we were updating it
without the kqueue lock held. For EVFILT_TIMER knotes, there is no
knlist lock, so the knote activation could occur during the kn_status
update and result in KN_QUEUED being lost, in which case we'd enqueue
an already-enqueued knote, corrupting the queue.

Fix the problem by setting or clearing KN_DISABLED before dropping the
kqueue lock to call into the filter. KN_DISABLED is used only by the
core kevent code, so there is no side effect from setting it earlier.

Reported and tested by: Sylvain GALLIANO <sg@efficientip.com>
Reviewed by: kib
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18060