UBIFS: fix LEB list freeing

When freeing the c->idx_lebs list, we have to release the LEBs as well,
because we might be called from mount to read-only mode code. Otherwise
the LEBs stay taken forever, which may cause problems when we re-mount
back ro RW mode.

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