vfs: fix applying modtime for an open Write Handle

The symptom of this was that the time set when the file was open was
lost.  This was causing one of the mount tests to fail too.

vfs/write.go

@@ -181,7 +181,10 @@ func (fh *WriteFileHandle) close() (err error) {
 	}
 	fh.closed = true
 	// leave writer open until file is transferred
-	defer fh.file.delWriter(fh, false)
+	defer func() {
+		fh.file.delWriter(fh, false)
+		fh.file.finishWriterClose()
+	}()
 	// If file not opened and not safe to truncate then then leave file intact
 	if !fh.opened && !fh.safeToTruncate() {
 		return nil