ip6_gre: Avoid updating tunnel->tun_hlen in __gre6_xmit()

ip6_gre: Avoid updating tunnel->tun_hlen in __gre6_xmit()

[ Upstream commit 397f440834f3e8c014c770fbf1d238c9af7ad31b ]

Do not update tunnel->tun_hlen in data plane code.  Use a local variable
instead, just like "tunnel_hlen" in net/ipv4/ip_gre.c:gre_fb_xmit().

Co-developed-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Peilin Ye <peilin.ye@bytedance.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>