ip6_gre: fix a pontential issue in ip6erspan_rcv

pskb_may_pull() can change skb->data, so we need to load ipv6h/ershdr at
the right place.

Fixes: 5a963eb6 ("ip6_gre: Add ERSPAN native tunnel support")
Cc: William Tu <u9012063@gmail.com>
Acked-by: William Tu <u9012063@gmail.com>
Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_gre.c

@@ -507,12 +507,11 @@ static int ip6erspan_rcv(struct sk_buff *skb, int gre_hdr_len,
 	struct ip6_tnl *tunnel;
 	u8 ver;
 
-	ipv6h = ipv6_hdr(skb);
-	ershdr = (struct erspan_base_hdr *)skb->data;
-
 	if (unlikely(!pskb_may_pull(skb, sizeof(*ershdr))))
 		return PACKET_REJECT;
 
+	ipv6h = ipv6_hdr(skb);
+	ershdr = (struct erspan_base_hdr *)skb->data;
 	ver = (ntohs(ershdr->ver_vlan) & VER_MASK) >> VER_OFFSET;
 	tpi->key = cpu_to_be32(ntohs(ershdr->session_id) & ID_MASK);