ipv6: Fix sporadic sendmsg -EINVAL when sending to multicast groups.

Thanks to excellent diagnosis by Eduard Guzovsky.

The core problem is that on a network with lots of active
multicast traffic, the neighbour cache can fill up.  If
we try to allocate a new route and thus neighbour cache
entry, the bog-standard GC attempt the neighbour layer does
in ineffective because route entries hold a reference
to the existing neighbour entries and GC can only liberate
entries with no references.

IPV4 already has a way to handle this, by doing a route cache
GC in such situations (when neigh attach returns -ENOBUFS).

So simply mimick this on the ipv6 side.

Tested-by: Eduard Guzovsky <eguzovsky@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ndisc.h

@@ -155,9 +155,9 @@ static inline struct neighbour * ndisc_get_neigh(struct net_device *dev, const s
 {
 
 	if (dev)
-		return __neigh_lookup(&nd_tbl, addr, dev, 1);
+		return __neigh_lookup_errno(&nd_tbl, addr, dev);
 
-	return NULL;
+	return ERR_PTR(-ENODEV);
 }