sit: reload iphdr in ipip6_rcv

sit: reload iphdr in ipip6_rcv

[ Upstream commit b699d0035836f6712917a41e7ae58d84359b8ff9 ]

Since iptunnel_pull_header() can call pskb_may_pull(),
we must reload any pointer that was related to skb->head.

Fixes: a09a4c8dd1ec ("tunnels: Remove encapsulation offloads on decap")
Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>