net: ipv6: send unsolicited NA on admin up

ndisc_notify is the ipv6 equivalent to arp_notify. When arp_notify is
set to 1, gratuitous arp requests are sent when the device is brought up.
The same is expected when ndisc_notify is set to 1 (per ndisc_notify in
Documentation/networking/ip-sysctl.txt). The NA is not sent on NETDEV_UP
event; add it.

Fixes: 5cb04436 ("ipv6: add knob to send unsolicited ND on link-layer address change")
Signed-off-by: NDavid Ahern <dsa@cumulusnetworks.com>
Acked-by: NHannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ndisc.c

@@ -1748,6 +1748,8 @@ static int ndisc_netdev_event(struct notifier_block *this, unsigned long event,
 	case NETDEV_CHANGEADDR:
 		neigh_changeaddr(&nd_tbl, dev);
 		fib6_run_gc(0, net, false);
+		/* fallthrough */
+	case NETDEV_UP:
 		idev = in6_dev_get(dev);
 		if (!idev)
 			break;