decnet: Use rcu_barrier() on module unload.

The decnet module unloading as been disabled with a '#if 0' statement,
because it have had issues.

We add a rcu_barrier() anyhow for correctness.

The maintainer (Chrissie Caulfield) will look into the unload issue
when time permits.

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