vfs: fix directory cache serving stale data

The VFS directory cache layer didn't update directory entry properties
if they are reused after cache invalidation.

Update them unconditionally as newDir sets them to the same value and
setting a pointer is cheaper in both LoC as well as CPU cycles than a
branch.

Also add a test exercising this behavior.

Fixes #6335

vfs/dir.go

@@ -760,6 +760,7 @@ func (d *Dir) _readDirFromEntries(entries fs.DirEntries, dirTree dirtree.DirTree
 			dir := node.(*Dir)
 			dir.mu.Lock()
 			dir.modTime = item.ModTime(context.TODO())
+			dir.entry = item
 			if dirTree != nil {
 				err = dir._readDirFromDirTree(dirTree, when)
 				if err != nil {