ethtool: add and use message type for tunnel info reply

Tunnel offload info code uses ETHTOOL_MSG_TUNNEL_INFO_GET message type (cmd
field in genetlink header) for replies to tunnel info netlink request, i.e.
the same value as the request have. This is a problem because we are using
two separate enums for userspace to kernel and kernel to userspace message
types so that this ETHTOOL_MSG_TUNNEL_INFO_GET (28) collides with
ETHTOOL_MSG_CABLE_TEST_TDR_NTF which is what message type 28 means for
kernel to userspace messages.

As the tunnel info request reached mainline in 5.9 merge window, we should
still be able to fix the reply message type without breaking backward
compatibility.

Fixes: c7d759eb ("ethtool: add tunnel info interface")
Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>