net/core: move vlan_depth out of while loop in skb_network_protocol()

[ Bug added added in commit 05e8ef4a (net: factor out
  skb_mac_gso_segment() from skb_gso_segment() ) ]

move vlan_depth out of while loop, or else vlan_depth always is ETH_HLEN,
can not be increased, and lead to infinite loop when frame has two vlan headers.
Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -2219,9 +2219,9 @@ struct sk_buff *skb_mac_gso_segment(struct sk_buff *skb,
 	struct sk_buff *segs = ERR_PTR(-EPROTONOSUPPORT);
 	struct packet_offload *ptype;
 	__be16 type = skb->protocol;
+	int vlan_depth = ETH_HLEN;
 
 	while (type == htons(ETH_P_8021Q)) {
-		int vlan_depth = ETH_HLEN;
 		struct vlan_hdr *vh;
 
 		if (unlikely(!pskb_may_pull(skb, vlan_depth + VLAN_HLEN)))