ipv4: ip_check_defrag should correctly check return value of skb_copy_bits

skb_copy_bits() returns zero on success and negative value on error,
so it is needed to invert the condition in ip_check_defrag().

Fixes: 1bf3751ec90c ("ipv4: ip_check_defrag must not modify skb before unsharing")
Signed-off-by: Alexander Drozdov <al.drozdov@gmail.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_fragment.c

@@ -664,7 +664,7 @@ struct sk_buff *ip_check_defrag(struct sk_buff *skb, u32 user)
 	if (skb->protocol != htons(ETH_P_IP))
 		return skb;
 
-	if (!skb_copy_bits(skb, 0, &iph, sizeof(iph)))
+	if (skb_copy_bits(skb, 0, &iph, sizeof(iph)) < 0)
 		return skb;
 
 	if (iph.ihl < 5 || iph.version != 4)