linuxkpi: Add <linux/kmsg_dump.h>

This header declares register/unregister functions to allow a piece of
code to tell what function to call in case of a panic. Several panic
handlers may be registered.

The DRM generic code started to use it in Linux 6.10 as part of the
panic handler.

Reviewed by: bz, christos
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D54492