NFSv4.1: protect destroying and nullifying bc_serv structure

When we are shutting down the client, we free the callback
server structure and then at a later pointer we free the
transport used by the client. Yet, it's possible that after
the callback server is freed, the transport receives a
backchannel request at which point we can dereferene freed
memory. Instead, do the freeing the bc server and nullying
bc_serv under the lock.

Signed-off-by: Olga Kornievskaia <okorniev@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

Author: Olga Kornievskaia

fs/nfs/callback.c

@@ -270,7 +270,7 @@ void nfs_callback_down(int minorversion, struct net *net, struct rpc_xprt *xprt)
 	if (cb_info->users == 0) {
 		svc_set_num_threads(serv, NULL, 0);
 		dprintk("nfs_callback_down: service destroyed\n");
-		svc_destroy(&cb_info->serv);
+		xprt_svc_destroy_nullify_bc(xprt, &cb_info->serv);
 	}
 	mutex_unlock(&nfs_callback_mutex);
 }