vxlan: add ipv6 support

This patch adds IPv6 support to vxlan device, as the new version
RFC already mentions it:

   http://tools.ietf.org/html/draft-mahalingam-dutt-dcops-vxlan-03



Cc: David Stevens <dlstevens@us.ibm.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>