can: af_can.c use rcu_barrier() on module unload.

This module uses rcu_call() thus it should use rcu_barrier()
on module unload.

Signed-off-by: Jesper Dangaard Brouer <hawk@comx.dk>
Acked-by: Oliver Hartkopp <oliver@hartkopp.net>
Signed-off-by: David S. Miller <davem@davemloft.net>