ipv4: Don't override return code from ip_route_input_noref()

After ip_route_input() calls ip_route_input_noref(), another
check on skb_dst() is done, but if this fails, we shouldn't
override the return code from ip_route_input_noref(), as it
could have been more specific (i.e. -EHOSTUNREACH).

This also saves one call to skb_dst_force_safe() and one to
skb_dst() in case the ip_route_input_noref() check fails.
Reported-by: Sabrina Dubroca <sdubroca@redhat.com>
Fixes: 9df16efa ("ipv4: call dst_hold_safe() properly")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Acked-by: Wei Wang <weiwan@google.com>
Acked-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/route.h

@@ -189,10 +189,11 @@ static inline int ip_route_input(struct sk_buff *skb, __be32 dst, __be32 src,
 
 	rcu_read_lock();
 	err = ip_route_input_noref(skb, dst, src, tos, devin);
-	if (!err)
+	if (!err) {
 		skb_dst_force_safe(skb);
-	if (!skb_dst(skb))
-		err = -EINVAL;
+		if (!skb_dst(skb))
+			err = -EINVAL;
+	}
 	rcu_read_unlock();
 
 	return err;