nfnetlink_queue: Use rcu_barrier() on module unload.

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

Also fixed a trivial typo 'nfetlink' -> 'nfnetlink' in comment.

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