compat: Backport ipv6_stub change

compat: Backport ipv6_stub change

A patch backported to the Linux stable 4.14 tree and present in the
latest stable 4.14.181 kernel breaks ipv6_stub usage.

The commit is
8ab8786f78c3 ("net ipv6_stub: use ip6_dst_lookup_flow instead of ip6_dst_lookup").

Create the compat layer define to check for it and fixup usage in vxlan
and geneve modules.

Passes Travis here:
https://travis-ci.org/github/gvrose8192/ovs-experimental/builds/689798733

Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: William Tu <u9012063@gmail.com>