Staging: batman-adv: wait for call_rcu before unloading module

synchronize_rcu respective synchronize_net only waits for the rcu grace
period to elapse and we may fail to finish the calls which were made to
call_rcu in that time. In result the module could be unloaded during the
execution of the RCU callbacks.

rcu_barrier[1] will now wait for all outstanding RCU callbacks to finish
before continuing.

[1] Documentation/RCU/rcubarrier.txt

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