UBIFS: support mounting of UBI volume character devices

This patch makes it possible to mount UBI character device
nodes, and use something like:

$ mount -t ubifs /dev/ubi_volume_name /mnt/ubifs

instead of the old restrictive 'nodev' semantics:

$ mount -t ubifs ubi0_0 /mnt/ubifs

[Comments and the patch were amended a bit by Artem]

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>