tcp: always set tcp_tun_port to a correct value

The tcp_tun_port field that is used to pass port value between UDP
and TCP in case of tunneling is a generic field that used to pass
data between network layers. It can be contaminated on entry, e.g.
by a VLAN tag set by a NIC driver. Explicily set it, so that it
is zeroed out in a normal not-tunneled TCP. If it contains garbage,
tcp_twcheck() later can enter wrong block of code and treat the packet
as incorrectly tunneled one. On main and stable/14 that will end up
with sending incorrect responses, but on stable/13 with ipfw(8) and
pcb-matching rules it may end up in a panic.

This is a minimal conservative patch to be merged to stable branches.
Later we may redesign this.

PR: 275169
Reviewed by: tuexen
Differential Revision: https://reviews.freebsd.org/D43065

(cherry picked from commit 513f2e2e7180202167ca2963d815d2a4c3ac0af9)