net: switchdev: Add extack to struct switchdev_notifier_info

In order to pass extack to the drivers that need it, add an extack field
to struct switchdev_notifier_info, and an extack argument to the
function call_switchdev_blocking_notifiers(). Also add a helper function
switchdev_notifier_info_to_extack().

Signed-off-by: Petr Machata <petrm@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Acked-by: Ivan Vecera <ivecera@redhat.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>