inpcb: Allow SO_REUSEPORT_LB to be used in jails

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

(cherry picked from commit d93ec8cb1324d04d7cae19fb7fa98ade2ff33c80)