LinuxKPI: add KPI for netdev_notifier_info returning ifp

While currently the ifp gets cast to a net_device and then returned
and consumers are expecting an ifp again, allow parallel usage now and
in the future by extending and also passing the ifp directly back in
the netdev_notifier_info. Add a function to return the ifp instead of
the net_device.

Sponsored by: The FreeBSD Foundation
MFC after: 10 days
Suggested by: hselasky
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D30522