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
MFC after: 2 weeks
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D30423