Retain offset compatibility with pre-12.0 dumps

[cem]

Summary

As a follow-up to r324965, which adds support for compressed kernel dumps,
readjust dump header members slightly to mostly preserve ABI (somewhat) with
earlier (11.x and older) dumps.

Test Plan

This change does burn 4 bytes of panicstr, but I think that is a worthwhile
tradeoff. An alternative approach that doesn't burn 4 bytes of panicstr would
be to misalign the 8-byte dumpextent field, which would also be fine with me.

[markj]

I think this is fine. The panic string buffer is bigger than it needs to be.

We already have the compile-time assert in all the MD dump files.

[cem]

In D15829#334743, @markj wrote:

I think this is fine. The panic string buffer is bigger than it needs to be.

We already have the compile-time assert in all the MD dump files.

Thanks! I'll remove the assert before commit.