udp_tunnel: add the ability to hard-code IANA VXLAN

mlx5 has the IANA VXLAN port (4789) hard coded by the device,
instead of being added dynamically when tunnels are created.

To support this add a workaround flag to struct udp_tunnel_nic_info.
Skipping updates for the port is fairly trivial, dumping the hard
coded port via ethtool requires some code duplication. The port
is not a part of any real table, we dump it in a special table
which has no tunnel types supported and only one entry.

This is the last known workaround / hack needed to convert
all drivers to the new infra.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>