Fix deadlock in kernfs.Filesystem.revalidateChildLocked

It was calling Dentry.InsertChild with the dentry's mutex
already locked.

Updates #1035

PiperOrigin-RevId: 286962742

pkg/sentry/fsimpl/kernfs/filesystem.go

@@ -122,7 +122,7 @@ func (fs *Filesystem) revalidateChildLocked(ctx context.Context, vfsObj *vfs.Vir
 			return nil, err
 		}
 		// Reference on childVFSD dropped by a corresponding Valid.
-		parent.InsertChild(name, childVFSD)
+		parent.insertChildLocked(name, childVFSD)
 	}
 	return childVFSD.Impl().(*Dentry), nil
 }

pkg/sentry/fsimpl/kernfs/kernfs.go

@@ -239,14 +239,22 @@ func (d *Dentry) destroy() {
 //
 // Precondition: d must represent a directory inode.
 func (d *Dentry) InsertChild(name string, child *vfs.Dentry) {
+	d.dirMu.Lock()
+	d.insertChildLocked(name, child)
+	d.dirMu.Unlock()
+}
+
+// insertChildLocked is equivalent to InsertChild, with additional
+// preconditions.
+//
+// Precondition: d.dirMu must be locked.
+func (d *Dentry) insertChildLocked(name string, child *vfs.Dentry) {
 	if !d.isDir() {
 		panic(fmt.Sprintf("InsertChild called on non-directory Dentry: %+v.", d))
 	}
 	vfsDentry := d.VFSDentry()
 	vfsDentry.IncRef() // DecRef in child's Dentry.destroy.
-	d.dirMu.Lock()
 	vfsDentry.InsertChild(child, name)
-	d.dirMu.Unlock()
 }
 
 // The Inode interface maps filesystem-level operations that operate on paths to