net: gso: add HBH extension header offload support

This commit adds net_offload to IPv6 Hop-by-Hop extension headers (as it
is done for routing and dstopts) since it is supported in GSO and GRO.
This allows to remove specific HBH conditionals in GSO and GRO when
pulling and parsing an incoming packet.

Signed-off-by: Richard Gobert <richardbgobert@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/d4f8825a-1d55-4b12-9d67-a254dbbfa6ae@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>