Some tunnel protocols, such as Geneve, have a bit in the tunnel
header to indicate that it is an OAM packet. This means that the
packet should be processed as a tunnel control frame and not be
passed onto connected links.

When BFD is used inside of a tunnel it is often used in this control
capacity, so this adds an option to enable marking the outer header
when the output port is a tunnel that supports the OAM concept. It is
also possible to use tunnels as point-to-point links that are simply
carrying BFD as payload, so this is not always turned on.

Conceptually, this may also apply to other types of packets locally
generated by the switch, most obviously CFM. However, BFD seems to
be most commonly used for this type of tunnel monitoring application
so this only adds the option to BFD for the time being to avoid
unnecessarily adding configuration knobs that might never get used.

Signed-off-by: Jesse Gross <jesse@kernel.org>
Acked-by: Pravin B Shelar <pshelar@ovn.org>