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.

Sponsored by: The FreeBSD Foundation
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D29365

(cherry picked from commit fdcfe8a298e23bef9588cafad2672e0c5f48a327)