Retain offset compatibility with pre-12.0 dumps

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

Reviewed by: markj
X-MFC-With: r324965
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D15829