vfs: fix inefficient directory caching when directory reads are slow

Before this change, when querying directories with large datasets, if
the query duration exceeded the directory cache expiration time, the
cache became invalid by the time results were retrieved. This means
every execution of `_readDir` triggers `_readDirFromEntries`,
resulting in prolonged processing times.

After this change we update the directory time with the time at the
end of the query.

vfs/dir.go

@@ -579,7 +579,7 @@ func (d *Dir) _readDir() error {
 		return err
 	}
 
-	d.read = when
+	d.read = time.Now()
 	d.cleanupTimer.Reset(time.Duration(d.vfs.Opt.DirCacheTime * 2))
 
 	return nil