LinuxKPI: netdevice notifier callback argument

Introduce struct netdev_notifier_info as a container to pass
net_device to the callback functions.
Adjust netdev_notifier_info_to_dev() to return the net_device field.

Add explicit casts from ifp to ni->dev even though currently
struct net_device is defined to struct ifnet. This is needed in
preparation for untangling this and improving the net_device compat
code.

Obtained-from: bz_iwlwifi
Sponsored-by: The FreeBSD Foundation
MFC-after: 2 weeks
Reviewed-by: hselasky
Differential Revision: https://reviews.freebsd.org/D29365