Staging: batman-adv: Use synchronize_rcu instead of call_rcu

It is recommended [1] to use synchronize_rcu to simplify the code -
especially when otherwise extra locking is needed to protect other code
from picking stale elements. It also protects us for emitting to many
callbacks which may results in OOM conditions.

The only reason not to use it, would be in performance critical sections
or when we are not allowed to block.

[1] Documentation/RCU/checklist.txt

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