mutex.
- Don't use callout_drain() to stop the toffhandle callout while holding the
fdc mutex (this could deadlock) in functions called from softclock
(callouts aren't allowed to do voluntary sleeps). Instead, use
callout_stop(). Note that since we hold the associated mutex and are now
using callout_init_mtx(), callout_stop() is just as effective as
callout_drain(). (Though callout_drain() is still needed in detach to
make sure softclock isn't contesting on our mutex before we destroy the
mutex.)
- Remove unused callout 'tohandle' from softc.
MFC after: 1 week
9fc9d84b9b