Staging: batman-adv: Track references of batman_if in set_primary_if

set_primary_if exchanges the current primary interfaces with a new one.
This is a new reference and thus we have to count it and decrease the
count of the old primary interface.

Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>