gro: small napi_get_frags() optim

Remove one useless conditional branch :
napi->skb is NULL, so nothing bad can happen.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -3981,8 +3981,7 @@ struct sk_buff *napi_get_frags(struct napi_struct *napi)
 
 	if (!skb) {
 		skb = netdev_alloc_skb_ip_align(napi->dev, GRO_MAX_HEAD);
-		if (skb)
-			napi->skb = skb;
+		napi->skb = skb;
 	}
 	return skb;
 }