This patch adds support to configure foo-over-udp (FOU) and Generic
UDP Encapsulation for GRE tunnels. This configuration allows selection
of FOU or GUE for the tunnel, specification of the source and
destination ports for UDP tunnel, and enabling TX checksum. This
configuration only affects the transmit side of a tunnel.

Example:

ip link add name tun1 type gre remote 192.168.1.1 local 192.168.1.2 \
   ttl 225 encap fou encap-sport auto encap-dport 7777 encap-csum

This would create an GRE tunnel in GUE encapsulation where the source
port is automatically selected (based on hash of inner packet) and
checksums in the encapsulating UDP header are enabled.
Signed-off-by: Tom Herbert <therbert@google.com>