inpcb: use internal flag to mark pcbs that are inserted into lbgroup

Using INP_REUSEPORT_LB is unsafe, as it is basically a copy of socket's
SO_REUSEPORT_LB flag, which can be cleared by userland after bind().

Reviewed by: markj
Reported by: syzbot+e7d2e451f89fb444319b@syzkaller.appspotmail.com
Differential Revision: https://reviews.freebsd.org/D41197