Support genetlink families using simple fixed headers.
Assume fixed header is identical for all ops of the family for now.

Fixed headers are added to the request and reply structs as a _hdr
member, and copied to/from netlink messages appropriately.
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20231213231432.2944749-4-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>