batman-adv: Fix duplicated OGMs on NETDEV_UP

The state of slave interfaces are handled differently depending on whether
the interface is up or not. All active interfaces (IFF_UP) will transmit
OGMs. But for B.A.T.M.A.N. IV, also non-active interfaces are scheduling
(low TTL) OGMs on active interfaces. The code which setups and schedules
the OGMs must therefore already be called when the interfaces gets added as
slave interface and the transmit function must then check whether it has to
send out the OGM or not on the specific slave interface.

But the commit f0d97253 ("batman-adv: remove ogm_emit and ogm_schedule
API calls") moved the setup code from the enable function to the activate
function. The latter is called either when the added slave was already up
when batadv_hardif_enable_interface processed the new interface or when a
NETDEV_UP event was received for this slave interfac. As result, each
NETDEV_UP would schedule a new OGM worker for the interface and thus OGMs
would be send a lot more than expected.

Fixes: f0d97253

 ("batman-adv: remove ogm_emit and ogm_schedule API calls")
Reported-by: Linus Lüssing <linus.luessing@c0d3.blue>
Tested-by: Linus Lüssing <linus.luessing@c0d3.blue>
Acked-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>