gro: small napi_get_frags() optim

Remove one useless conditional branch :
napi->skb is NULL, so nothing bad can happen.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/core/dev.c b/net/core/dev.c
index ba3b7ea..c980524 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3981,8 +3981,7 @@
 
 	if (!skb) {
 		skb = netdev_alloc_skb_ip_align(napi->dev, GRO_MAX_HEAD);
-		if (skb)
-			napi->skb = skb;
+		napi->skb = skb;
 	}
 	return skb;
 }