Revert the optimization from r304436

r304436 attempted to optimize the handling of incoming UDP packet by only
making an expensive call to in_broadcast() if the mbuf was marked as an
broadcast packet. Unfortunately, this cannot work in the case of point-to-
point L2 protocols like PPP, which have no notion of "broadcast". The
optimization has been disabled for several months now with no progress
towards fixing it, so it needs to go.