ip6_gre: reload ipv6h in prepare_ip6gre_xmit_ipv6

[ Upstream commit 3bc817d665ac6d9de89f59df522ad86f5b5dfc03 ]

Since ip6_tnl_parse_tlv_enc_lim() can call pskb_may_pull()
which may change skb->data, so we need to re-load ipv6h at
the right place.

Fixes: 898b2979 ("ip6_gre: Refactor ip6gre xmit codes")
Cc: William Tu <u9012063@gmail.com>
Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Acked-by: William Tu <u9012063@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>