Add an 'ubi_open_volume_path(path, mode)' function which works like
'open_bdev_exclusive(path, mode, ...)' where path is the special file
representing the UBI volume, typically /dev/ubi0_0.

This is needed to teach UBIFS being able to mount UBI character devices.

[Comments and the patch were amended a bit by Artem]
Signed-off-by: NCorentin Chary <corentincj@iksaif.net>
Signed-off-by: NArtem Bityutskiy <Artem.Bityutskiy@nokia.com>