Support locking of directory entries

Moves the locking algorithm from file entry to file system entry.

Fixes whatwg#137

index.bs

@@ -135,9 +135,10 @@ To <dfn for="file system entry/lock">take a lock</dfn> with a |value| of
 
 1. Let |lock| be the |entry|'s [=file system entry/lock=].
 1. Let |count| be the |entry|'s [=file system entry/shared lock count=].
-1. Let |descendantLockStatus| be the result of [=file system entry/lock/checking for a descendant lock=] on |entry|.
-1. If |descendantLockStatus| is "`taken`":
-   1. Return "`failure`".
+1. If |entry|  is a [=directory entry=]:
+   1. Let |descendantLockStatus| be the result of [=file system entry/lock/checking for a descendant lock=] on |entry|.
+   1. If |descendantLockStatus| is "`taken`":
+     1. Return "`failure`".
 1. If |value| is "`exclusive`":
    1. If |lock| is "`open`":
      1. Set lock to "`taken-exclusive`".
@@ -158,16 +159,16 @@ Note: These steps have to be run on the [=file system queue=].
 
 <div algorithm>
 To <dfn for="file system entry/lock">check for a descendant lock</dfn> on a given
-[=/file system entry=] |entry|:
-
-1. Let |lock| be the |entry|'s [=file system entry/lock=].
-1. If |lock| is not "`open`":
-   1. Return "`taken`".
-1. If |entry| is a [=directory entry=]:
-   1. [=set/For each=] |child| of |entry|'s [=directory entry/children=]:
-      1. Let |descendantLockStatus| be the result of [=file system entry/lock/checking for a descendant lock=] on |child|.
-      1. If |descendantLockStatus| is "`taken`":
-         1. Return "`taken`".
+[=directory entry=] |directory|:
+
+1. [=set/For each=] |child| of |directory|'s [=directory entry/children=]:
+  1. Let |lock| be the |child|'s [=file system entry/lock=].
+  1. If |lock| is not "`open`":
+    1. Return "`taken`".
+  1. If |child| is a [=directory entry=]:
+    1. Let |descendantLockStatus| be the result of [=file system entry/lock/checking for a descendant lock=] on |child|.
+    1. If |descendantLockStatus| is "`taken`":
+      1. Return "`taken`".
 1. Return "`open`".
 
 Note: These steps have to be run on the [=file system queue=].