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

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/2018/03/15/62

Signed-off-by: Heiner Kallweit <[email protected]>
Tested-by: Geert Uytterhoeven <[email protected]>
Reviewed-by: Florian Fainelli <[email protected]>
Signed-off-by: David S. Miller <[email protected]>