net: check before dereferencing netdev_ops during busy poll

[Matteo Croce]

Hi,

Is there any reason for this panic fix not being applied in stable?

https://lore.kernel.org/netdev/20180313053248.13654-1-jelsas...@appneta.com/T/

It seems that linux 4.9.184 has the bug too.

Regards,
-- 
Matteo Croce
per aspera ad upstream