nfs: do not start the callback thread until we set rqstp->rq_task

This fixes an Oopsable race when starting up the callback server.
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Reviewed-by: Jeff Layton <jlayton@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfs/callback.c

@@ -235,7 +235,7 @@ static int nfs_callback_start_svc(int minorversion, struct rpc_xprt *xprt,
 
 	cb_info->serv = serv;
 	cb_info->rqst = rqstp;
-	cb_info->task = kthread_run(callback_svc, cb_info->rqst,
+	cb_info->task = kthread_create(callback_svc, cb_info->rqst,
 				    "nfsv4.%u-svc", minorversion);
 	if (IS_ERR(cb_info->task)) {
 		ret = PTR_ERR(cb_info->task);
@@ -245,6 +245,7 @@ static int nfs_callback_start_svc(int minorversion, struct rpc_xprt *xprt,
 		return ret;
 	}
 	rqstp->rq_task = cb_info->task;
+	wake_up_process(cb_info->task);
 	dprintk("nfs_callback_up: service started\n");
 	return 0;
 }