nfsd: Fix nfsd leaks sunrpc module references

Stefan Hajnoczi reports,
nfsd leaks 3 references to the sunrpc module here:

  # echo -n "asdf 1234" >/proc/fs/nfsd/portlist
  bash: echo: write error: Protocol not supported

Now stop nfsd and try unloading the kernel modules:

  # systemctl stop nfs-server
  # systemctl stop nfs
  # systemctl stop proc-fs-nfsd.mount
  # systemctl stop var-lib-nfs-rpc_pipefs.mount
  # rmmod nfsd
  # rmmod nfs_acl
  # rmmod lockd
  # rmmod auth_rpcgss
  # rmmod sunrpc
  rmmod: ERROR: Module sunrpc is in use
  # lsmod | grep rpc
  sunrpc                315392  3

It is caused by nfsd don't cleanup rpcb program for nfsd
when destroying svc service after creating xprt fail.
Reported-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfssvc.c

@@ -378,14 +378,13 @@ static void nfsd_last_thread(struct svc_serv *serv, struct net *net)
 	 * write_ports can create the server without actually starting
 	 * any threads--if we get shut down before any threads are
 	 * started, then nfsd_last_thread will be run before any of this
-	 * other initialization has been done.
+	 * other initialization has been done except the rpcb information.
 	 */
+	svc_rpcb_cleanup(serv, net);
 	if (!nn->nfsd_net_up)
 		return;
-	nfsd_shutdown_net(net);
-
-	svc_rpcb_cleanup(serv, net);
 
+	nfsd_shutdown_net(net);
 	printk(KERN_WARNING "nfsd: last server has exited, flushing export "
 			    "cache\n");
 	nfsd_export_flush(net);