if_gif: Use the MTU on the gif interface for the outgoing packets

[meta]

Summary

This is a revised version of D51866 to address review comments
and resolve conflicts after D51835.

As I mentioned in D51866 [1], I would like this changes to be included in
15.0-RELEASE. I worked on D51297 to ensure that the fix for bug 280736 would
be included in the release, as there has been no progress on D45854 for over
a year. Since this is a better approach for bug 280736, it should be included
as well.

Ongoing work can be seen on GitHub:
https://github.com/metalefty/freebsd-src/commits/gif/

[1] https://reviews.freebsd.org/D51866#1196328

[meta]

This worked very well for me, both for EtherIP (Ethernet-ovet-IPv6) and IPIP (v6-over-v6).

[zlei]

In D52594#1201880, @meta wrote:

This worked very well for me, both for EtherIP (Ethernet-ovet-IPv6) and IPIP (v6-over-v6).

I'm preparing the tests. It appears good.

[zlei]

I'm going to firstly integrate part of this change, to be precisely, the if_bridge's MTU change behavior of adding if_gif(4) as a member interface.

[hrs]

Please stop duplicating the existing review to obtain approval from someone else by excluding the original author.

[meta]

Nothing had happened for over a year since D45854 was submitted. I just tried to get things moving to avoid waiting another year. Because we didn't get feedback from you after the reviewers left review comments, and the deadline was approaching.

To be clear, I did not intend to exclude you, just forgot to add.