net: ipv6: Send Neighbor Solicitations in PROBE state as unicast

According to RFC 4861, ch. 7.3.3:

 "Upon entering the PROBE state, a node sends a unicast Neighbor
  Solicitation message to the neighbor using the cached link-layer
  address."

Zephyr's implementation was not compliant with behavior, as instead of
sending a unicast probe for reachability confirmation, it was sending a
multicast packet instead. This commit fixes it.

Signed-off-by: Robert Lubos <[email protected]>
(cherry picked from commit 8cd213e)

Author: rlubos

subsys/net/ip/ipv6_nbr.c

@@ -1549,7 +1549,7 @@ static void ipv6_nd_reachable_timeout(struct k_work *work)
 					data->ns_count);
 
 				ret = net_ipv6_send_ns(nbr->iface, NULL, NULL,
-						       NULL, &data->addr,
+						       &data->addr, &data->addr,
 						       false);
 				if (ret < 0) {
 					NET_DBG("Cannot send NS (%d)", ret);