Protect calls to mtod() from accessing NULL mbufs

[gnn]

Summary

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=203409

Test Plan

Trace all debug output using a modified tcpdebug script that looks at all sockets

[hiren]

Is it possible to handle 'm' being null in TCP_PROBE3() instead of these if/else checks everywhere?

[gnn]

I could make an uglier macro. If that would please the audience.

[hiren]

In D9035#186666, @gnn wrote:

I could make an uglier macro. If that would please the audience.

Hah. Uglier macro in one place might probably be better?

I'll see what others have to say. :-)

[mjg]

Have you checked generated assembly? This likely executes a branch regardless of whether SDT is enabled. On the other putting the check in the macro should get around that.

[gnn]

We actually don't need mtod() at all. If you want the mbuf, use the mbuf provider.