net: gro: remove is_ipv6 from napi_gro_cb

net: gro: remove is_ipv6 from napi_gro_cb

Remove is_ipv6 from napi_gro_cb and use sk->sk_family instead.
This frees up space for another ip_fixedid bit that will be added
in the next commit.

udp_sock_create always creates either a AF_INET or a AF_INET6 socket,
so using sk->sk_family is reliable. In IPv6-FOU, cfg->ipv6_v6only is
always enabled.

Signed-off-by: Richard Gobert <richardbgobert@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250923085908.4687-2-richardbgobert@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>