mtd: ubi: in case of failed fm, re-erase free blocks

In case of unclean reboot, one can't be sure that blocks considered free
were properly erased. In such case, we re-erase all free pebs.

Change-Id: I48b1fb5549ad97e69024814ae78a4abd50281ce1
Signed-off-by: Tanya Brokhman <tlinder@codeaurora.org>