NFS: get module in idmap PipeFS notifier callback

This is bug fix.
Notifier callback is called from SUNRPC module. So before dereferencing NFS
module we have to make sure, that it's alive.
Signed-off-by: NStanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/idmap.c

@@ -554,12 +554,16 @@ static int rpc_pipefs_event(struct notifier_block *nb, unsigned long event,
 	struct nfs_client *clp;
 	int error = 0;
 
+	if (!try_module_get(THIS_MODULE))
+		return 0;
+
 	while ((clp = nfs_get_client_for_event(sb->s_fs_info, event))) {
 		error = __rpc_pipefs_event(clp, event, sb);
 		nfs_put_client(clp);
 		if (error)
 			break;
 	}
+	module_put(THIS_MODULE);
 	return error;
 }