UBIFS: create the name of the background thread in every case

If the ubifs partition is mounted RO and then remounted RW we end
up with no thread name in ubifs_remount_rw() and the thread appears
nameless.

Signed-off-by: Sebastian Siewior <bigeasy@linutronix.de>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>