Eliminate rmlock from ipfw's BPF code.

After r343631 pfil hooks are invoked in net_epoch_preempt section,
this allows to avoid extra locking. Add NET_EPOCH_ASSER() assertion
to each ipfw_bpf_*tap*() call to require to be called from inside
epoch section.

Use NET_EPOCH_WAIT() in ipfw_clone_destroy() to wait until it becomes
safe to free() ifnet. And use on-stack ifnet pointer in each
ipfw_bpf_*tap*() call to avoid NULL pointer dereference in case when
V_*log_if global variable will become NULL during ipfw_bpf_*tap*() call.

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