inpcb: Allow SO_REUSEPORT_LB to be used in jails

[markj]

Summary

Currently SO_REUSEPORT_LB silently does nothing when set by a jailed
process. It is trivial to support this option in VNET jails, but it's
also useful in traditional jails where the semantics are not quite
clear.

This patch enables LB groups in jails with the following semantics:

- all PCBs in a group must belong to the same jail
- PCB lookup prefers jailed groups to non-jailed groups, exactly how
we handle individual sockets

One pre-existing quirk of lbgroups is that they are matched before
jailed sockets. This seems like a (minor?) bug to me, but that
behaviour is preserved with this change.

[glebius]

Didn't do a thorough review, but overall looks good. Thanks!

sys/netinet/in_pcb.c

280

Why is this M_NOWAIT spread around the LB group code? This all happens in context of setsockopt(2), doesn't it?

[markj]

sys/netinet/in_pcb.c

280

No, it's a bit weird. It happens when binding the socket. First you set the SO_REUSEPORT_LB option, then bind. All sockets with SO_REUSEPORT_LB which bind to the same port are added to the same LB group.

Here, the inpcb and hash table are locked, so we can't allocate anything with M_WAITOK. I don't like it, I agree that we should be using M_WAITOK here but I haven't thought about how to fix it.