gre: Add netlink interface

This patch adds a netlink interface that will eventually displace
the existing ioctl interface.  It utilises the elegant rtnl_link_ops
mechanism.

This also means that user-space no longer needs to rely on the
tunnel interface being of type GRE to identify GRE tunnels.  The
identification can now occur using rtnl_link_ops.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>