UBIFS: do not lie about used blocks

Do not force UBIFS return 0 used space when it is empty. It leads
to a situation when creating any file immediately produces tens of
used blocks, which looks very weird. It is better to be honest and
say that some blocks are used even if the FS is empty. And ext2
does the same.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>