NFSv4: Ensure nfs_callback_down() calls svc_destroy()

Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/callback.c

@@ -73,6 +73,7 @@ static void nfs_callback_svc(struct svc_rqst *rqstp)
 		svc_process(serv, rqstp);
 	}
 
+	svc_exit_thread(rqstp);
 	nfs_callback_info.pid = 0;
 	complete(&nfs_callback_info.stopped);
 	unlock_kernel();