sunrpc: Use rcu_barrier() on unload.

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

Have not verified that the possibility for new call_rcu() callbacks
has been disabled.  As a hint for checking, the functions calling
call_rcu() (unx_destroy_cred and generic_destroy_cred) are
registered as crdestroy function pointer in struct rpc_credops.
Acked-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: NTrond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: NJesper Dangaard Brouer <hawk@comx.dk>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sunrpc/sunrpc_syms.c

@@ -66,6 +66,7 @@ cleanup_sunrpc(void)
 #ifdef CONFIG_PROC_FS
 	rpc_proc_exit();
 #endif
+	rcu_barrier(); /* Wait for completion of call_rcu()'s */
 }
 MODULE_LICENSE("GPL");
 module_init(init_sunrpc);