clockintr: protect struct clockintr_queue with a mutex

clockintr: protect struct clockintr_queue with a mutex

Add a mutex (cq_mtx) to stuct clockintr_queue so that arbitrary CPUs
can manipulate clock interrupts established on arbitrary CPU queues.

Refactor the bulk of clockintr_schedule() into clockintr_schedule_locked()
so we can reuse it from within the mutex.

Tested by mlarkin@.  Neat bug found by mlarkin@.  With tweaks from
kettenis@.

ok kettenis@