net: Add info for NETDEV_CHANGEUPPER event

Some consumers of NETDEV_CHANGEUPPER event would like to know which
upper device was linked/unlinked and what operation was carried.

Add information in the notifier info block for that purpose.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>