inpcb: Allow SO_REUSEPORT_LB to be used in jails

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.

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

This is a straightforward extension of the semantics used for individual
listening sockets. One pre-existing quirk of the lbgroup implementation
is that non-jailed lbgroups are searched before jailed listening
sockets; that is preserved with this change.

Discussed with: glebius
MFC after: 1 month
Sponsored by: Modirum MDPay
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D37029