MFC r266800 by vanhu:
IPv4-in-IPv6 and IPv6-in-IPv4 IPsec tunnels.
For IPv6-in-IPv4, you may need to do the following command
on the tunnel interface if it is configured as IPv4 only:
ifconfig <interface> inet6 -ifdisabled

Code logic inspired from NetBSD.
PR: kern/169438

MC r266822 by bz:
Use IPv4 statistics in ipsec4_process_packet() rather than the IPv6
version. This also unbreaks the NOINET6 builds after r266800.

MFC r268083 by zec:
The assumption in ipsec4_process_packet() that the payload may be
only IPv4 is wrong, so check the IP version before mangling the
payload header.

MFC r272394:
Do not strip outer header when operating in transport mode.
Instead requeue mbuf back to IPv4 protocol handler. If there is one extra IP-IP
encapsulation, it will be handled with tunneling interface. And thus proper
interface will be exposed into mbuf's rcvif. Also, tcpdump that listens on tunneling
interface will see packets in both directions.

PR: 194761

MFC r266800 by vanhu:
IPv4-in-IPv6 and IPv6-in-IPv4 IPsec tunnels.
For IPv6-in-IPv4, you may need to do the following command
on the tunnel interface if it is configured as IPv4 only:
ifconfig <interface> inet6 -ifdisabled

Code logic inspired from NetBSD.
PR: kern/169438

MC r266822 by bz:
Use IPv4 statistics in ipsec4_process_packet() rather than the IPv6
version. This also unbreaks the NOINET6 builds after r266800.

MFC r268083 by zec:
The assumption in ipsec4_process_packet() that the payload may be
only IPv4 is wrong, so check the IP version before mangling the
payload header.

MFC r272394:
Do not strip outer header when operating in transport mode.
Instead requeue mbuf back to IPv4 protocol handler. If there is one extra IP-IP
encapsulation, it will be handled with tunneling interface. And thus proper
interface will be exposed into mbuf's rcvif. Also, tcpdump that listens on tunneling
interface will see packets in both directions.

PR: 194761