Maciej reported that when using vxlan packets we can see a TTL of 64
in the packet while for geneve packets the TTL is set to 0.

This is a kernel issue in that vxlan driver in its vxlan_xmit_one()
routine derives TTL from the route if otherwise not explicitly set
(such as by BPF tunnel key):

  ttl = ttl ? : ip4_dst_hoplimit(&rt->dst);

In geneve driver however, geneve_xmit_skb() only does the above in
non-collect_md mode, which means, if not explicitly set, the TTL will
remain 0 here.

I'll post a kernel fix separately, but simple workaround is to just
set TTL in BPF tunnel key to fixed value of 64.

tunnel_ttl is part of 4.9 bpf uapi header, so there are no issues with
backwards compat.

Reported-by: Maciej Skrocki <maciejskrocki@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>