ip link: Add support for remote checksum offload to IP tunnels

ip link: Add support for remote checksum offload to IP tunnels

This patch adds support to remote checksum checksum offload
confinguration for IPIP, SIT, and GRE tunnels. This patch
adds a [no]encap-remcsum to ip link command which applicable
when configured tunnels that use GUE.

http://tools.ietf.org/html/draft-herbert-remotecsumoffload-00

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 \
   encap-remcsum

This would create an GRE tunnel in GUE encapsulation where the source
port is automatically selected (based on hash of inner packet),
checksums in the encapsulating UDP header are enabled (needed.for
remote checksum offload), and remote checksum ffload is configured to
be used on the tunnel (affects TX side).

Signed-off-by: Tom Herbert <therbert@google.com>