UBIFS uses a bdi device per volume, but does not care to hand out unique
names to each of them. This causes an error when trying to mount more
than one volumes. Append the UBI volume and device ID to avoid that.

[Amended a bit by Artem Bityutskiy]
Signed-off-by: NDaniel Mack <daniel@caiaq.de>
Cc: Artem Bityutskiy <dedekind@infradead.org>
Cc: Adrian Hunter <ext-adrian.hunter@nokia.com>
Cc: linux-mtd@lists.infradead.org
Signed-off-by: NArtem Bityutskiy <Artem.Bityutskiy@nokia.com>