ip6_tunnel: must reload ipv6h in ip6ip6_tnl_xmit()

[ Upstream commit 21b995a9cb093fff33ec91d7cb3822b882a90a1e ]

Since ip6_tnl_parse_tlv_enc_lim() can call pskb_may_pull(),
we must reload any pointer that was related to skb->head
(or skb->data), or risk use after free.

Fixes: c12b395a ("gre: Support GRE over IPv6")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Dmitry Kozlov <xeb@mail.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>