unix: Fix uipc_peeraddr() to handle self-connected sockets

Reported by: syzbot+c2da2dbae5fe006556bc@syzkaller.appspotmail.com
Reported by: syzbot+b4d6b093b1d78bfa859b@syzkaller.appspotmail.com
Fixes: e8f6e5b2d969 ("unix: Fix locking in uipc_peeraddr()")
MFC after: 1 week
Sponsored by: The FreeBSD Foundation