UBIFS: always clean up GC LEB space

When we mount UBIFS, GC LEB may contain out-of-date information,
and UBIFS should update lprops and set free space for thei LEB.
Currently UBIFS does this only if mounted R/W. But for R/O mount
we have to do the same, because otherwise we will have incorrect
FS free space reported to user-space.

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