net: linkwatch: add check for netdevice being present to linkwatch_do_dev

When bringing down the netdevice (incl. detaching it) and calling
netif_carrier_off directly or indirectly the latter triggers an
asynchronous linkwatch event.
This linkwatch event eventually may fail to access chip registers in
the ndo_get_stats/ndo_get_stats64 callback because the device isn't
accessible any longer, see call trace in [0].

To prevent this scenario don't check for IFF_UP only, but also make
sure that the netdevice is present.

[0] https://lists.openwall.net/netdev/2018/03/15/62Signed-off-by: NHeiner Kallweit <hkallweit1@gmail.com>
Tested-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/link_watch.c

@@ -155,7 +155,7 @@ static void linkwatch_do_dev(struct net_device *dev)
 	clear_bit(__LINK_STATE_LINKWATCH_PENDING, &dev->state);
 
 	rfc2863_policy(dev);
-	if (dev->flags & IFF_UP) {
+	if (dev->flags & IFF_UP && netif_device_present(dev)) {
 		if (netif_carrier_ok(dev))
 			dev_activate(dev);
 		else