ip link: initial support for bareudp devices

ip link: initial support for bareudp devices

Bareudp devices provide a generic L3 encapsulation for tunnelling
different protocols like MPLS, IP, NSH, etc. inside a UDP tunnel.

This patch is based on original work from Martin Varghese:
https://lore.kernel.org/netdev/1570532361-15163-1-git-send-email-martinvarghesenokia@gmail.com/

Examples:

  - ip link add dev bareudp0 type bareudp dstport 6635 ethertype mpls_uc

This creates a bareudp tunnel device which tunnels L3 traffic with
ethertype 0x8847 (unicast MPLS traffic). The destination port of the
UDP header will be set to 6635. The device will listen on UDP port 6635
to receive traffic.

  - ip link add dev bareudp0 type bareudp dstport 6635 ethertype ipv4 multiproto

Same as the MPLS example, but for IPv4. The "multiproto" keyword allows
the device to also tunnel IPv6 traffic.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>