Fix the design of ifq_dequeue/altq_dequeue by adding an mbuf pointer and

Fix the design of ifq_dequeue/altq_dequeue by adding an mbuf pointer and
requiring that a polled mbuf be passed as an argument to the dequeue
function.  Assert that the passed argument matches the mbuf that is actually
dequeued.  Also remove assignments of the return value from ifq_dequeue()
in such cases which implied that the mbuf might be different when, in fact,
it had better not be.