ipv6: If neigh lookup fails during icmp6 dst allocation, propagate error.

Don't just succeed with a route that has a NULL neighbour attached.
This follows the behavior of addrconf_dst_alloc().

Allowing this kind of route to end up with a NULL neigh attached will
result in packet drops on output until the route is somehow
invalidated, since nothing will meanwhile try to lookup the neigh
again.

A statistic is bumped for the case where we see a neigh-less route on
output, but the resulting packet drop is otherwise silent in nature,
and frankly it's a hard error for this to happen and ipv6 should do
what ipv4 does which is say something in the kernel logs.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -1089,8 +1089,10 @@ struct dst_entry *icmp6_dst_alloc(struct net_device *dev,
 		neigh_hold(neigh);
 	else {
 		neigh = __neigh_lookup_errno(&nd_tbl, &fl6->daddr, dev);
-		if (IS_ERR(neigh))
-			neigh = NULL;
+		if (IS_ERR(neigh)) {
+			dst_free(&rt->dst);
+			return ERR_CAST(neigh);
+		}
 	}
 
 	rt->dst.flags |= DST_HOST;