UBIFS: improve debugging lprops scanning a little

When doing the lprops extra check ('dbg_check_lprops()') we scan whole media.
We even scan empty and freeable LEBs which may contain garbage, which we handle
after scanning. This patch teach the lprops checking function
('scan_check_cb()') to avoid scanning for free and freeable LEBs and save time.

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