ip6_gre: Set flowi6_proto as IPPROTO_GRE in xmit path.

In gre6 xmit path, we are sending a GRE packet, so set fl6 proto
to IPPROTO_GRE properly.

Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: David S. Miller <davem@davemloft.net>