Support locking of directory entries

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

Fixes whatwg#137

index.bs

@@ -135,6 +135,9 @@ To <dfn for="file system entry/lock">take</dfn> a [=file system entry/lock=] wit
 
 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 |hasDescendantLock| be the result of [=file system entry/lock/checking for a descendant lock=] on |entry|.
+1. If |hasDescendantLock| is true:
+   1. Return "`failure`".
 1. If |value| is "`exclusive`":
    1. If |lock| is "`open`":
      1. Set lock to "`taken-exclusive`".
@@ -153,6 +156,24 @@ Note: These steps have to be run on the [=file system queue=].
 
 </div>
 
+<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 true.
+1. If |entry| is a [=directory entry=]:
+   1. [=set/For each=] |child| of |entry|'s [=directory entry/children=]:
+      1. Let |hasDescendantLock| be the result of [=file system entry/lock/checking for a descendant lock=] on |child|.
+      1. If |hasDescendantLock| is true:
+         1. Return true.
+1. Return false.
+
+Note: These steps have to be run on the [=file system queue=].
+
+</div>
+
 <div algorithm>
 To <dfn for="file system entry/lock">release</dfn> a [=file system entry/lock=] on a given
 [=/file system entry=] |entry|: