sunrpc/auth_gss: Call rcu_barrier() on module unload.

As the module uses rcu_call() we should make sure that all
rcu callback has been completed before removing the code.

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