Widen epoch coverage (step 2) + refactor PCB info locking (step 2).

[glebius]

Summary

This patch extends network epoch coverage on output path. Now
calling ip_output() and ip6_output() requires epoch. This
eliminates a bunch of epoch recursions, and what is important
it eliminates ALL epoch recursions happening on the fast path:
forwarding, TCP in/out, UDP in/out.

It also relaxes assertions in PCB lookup functions, since now
ipi_lock is almost unused, we rely on ipi_list_lock/ipi_hash_lock
for modifications and on the epoch for lookups.

The above two changes are somewhat tied together, thus a single
review. The commit plan is 14 commits, full history available
here:

https://github.com/glebius/FreeBSD/commits/net_epoch2

[bz]

Hi Glebius, I have a patch which tried to remove recursions within netinet6. I was down to one in December and since new ones have shown up. I am not sure if I'll be able to look at this one before next week but I'll try to upload my current work in progress in case that'll help you. I have EPOCH calls around all ip6_output() calls which this one equally adds, so most of my changes should be "downstream" from yours.

[glebius]

I'd really appreciate some reviewing. I'm going to commit this week.

[glebius]

Rebase to fresh master.