netinet: Tighten checks for unspecified source addresses

The assertions added in commit b0ccf53f2455 ("inpcb: Assert against
wildcard addrs in in_pcblookup_hash_locked()") revealed that protocol
layers may pass the unspecified address to in_pcblookup().

Add some checks to filter out such packets before we attempt an inpcb
lookup:

• Disallow the use of an unspecified source address in in_pcbladdr() and in6_pcbladdr().
• Disallow IP packets with an unspecified destination address.
• Disallow TCP packets with an unspecified source address, and add an assertion to verify the comment claiming that the case of an unspecified destination address is handled by the IP layer.

Reported by: syzbot+9ca890fb84e984e82df2@syzkaller.appspotmail.com
Reported by: syzbot+ae873c71d3c71d5f41cb@syzkaller.appspotmail.com
Reported by: syzbot+e3e689aba1d442905067@syzkaller.appspotmail.com
Reviewed by: glebius, melifaro
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Sponsored by: Modirum MDPay
Differential Revision: https://reviews.freebsd.org/D38570