be woken up by kthread_exit. This is racey and in some cases the kthread will
exit before ndis gets around to sleep so it will be stuck indefinitely. This
change reuses the kq_exit variable to indicate that the thread has gone and
will loop on tsleep with a timeout waiting for it. If the kthread has already
exited then it will not sleep at all.
Approved by: re (rwatson)