LinuxKPI: add devcoredump.h

Add linux/devcoredump.h with stub implementation of dev_coredumpv()
and dev_coredumpsg() which only free the passed in SG table as needed
for iwlwifi.

Sponsored by: The FreeBSD Foundation
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D30423

(cherry picked from commit 29923fea032a6ac7c0defd74d0b23468b16fda93)