UBIFS: avoid kernel error if ubifs superblock read fails

.get_sb is called on mounts with automatic fs detection too, so this
function should print an error if it cannot read the superblock in
debug mode only (new behaviour conforms the other fs types)

Signed-off-by: Steffen Sledz <sledz@dresearch.de>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>