bridge: Use rcu_barrier() instead of syncronize_net() on unload.

When unloading modules that uses call_rcu() callbacks, then we must
use rcu_barrier().  This module uses syncronize_net() which is not
enough to be sure that all callback has been completed.

Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Jesper Dangaard Brouer <hawk@comx.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>