ip: fix error handling in ip_finish_output2()

__neigh_create() returns either a pointer to struct neighbour or PTR_ERR().
But the caller expects it to return either a pointer or NULL.  Replace
the NULL check with IS_ERR() check.

The bug was introduced in a263b309
("ipv4: Make neigh lookups directly in output packet path.").
Signed-off-by: NVasily Kulikov <segoon@openwall.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_output.c

@@ -197,7 +197,7 @@ static inline int ip_finish_output2(struct sk_buff *skb)
 	neigh = __ipv4_neigh_lookup_noref(dev, nexthop);
 	if (unlikely(!neigh))
 		neigh = __neigh_create(&arp_tbl, &nexthop, dev, false);
-	if (neigh) {
+	if (!IS_ERR(neigh)) {
 		int res = dst_neigh_output(dst, neigh, skb);
 
 		rcu_read_unlock_bh();