ipv6: Initialize ip6_tnl.hlen in gre tunnel even if no route is found

The ip6_tnl.hlen (gre and ipv6 headers length) is independent from the
outgoing interface, so it would be better to initialize it even when no
route is found, otherwise its value will be zero.
While I'm not sure if this could happen in real life, but doing that
will avoid to call the skb_push function with a zero in ip6gre_header
function.

Suggested-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Oussama Ghorbel <ou.ghorbel@gmail.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>