vxlan: add ipv6 route short circuit support

route short circuit only has IPv4 part, this patch adds
the IPv6 part. nd_tbl will be needed.

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