gro: Only reset frag0 when skb can be pulled

Currently skb_gro_header_slow unconditionally resets frag0 and
frag0_len.  However, when we can't pull on the skb this leaves
the GRO fields in an inconsistent state.

This patch fixes this by only resetting those fields after the
pskb_may_pull test.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -1649,9 +1649,12 @@ static inline int skb_gro_header_hard(struct sk_buff *skb, unsigned int hlen)
 static inline void *skb_gro_header_slow(struct sk_buff *skb, unsigned int hlen,
 					unsigned int offset)
 {
+	if (!pskb_may_pull(skb, hlen))
+		return NULL;
+
 	NAPI_GRO_CB(skb)->frag0 = NULL;
 	NAPI_GRO_CB(skb)->frag0_len = 0;
-	return pskb_may_pull(skb, hlen) ? skb->data + offset : NULL;
+	return skb->data + offset;
 }
 
 static inline void *skb_gro_mac_header(struct sk_buff *skb)