mtd: ubi: modify scan_peb sysfs to scrub_all

One of the limitations of the NAND devices is the method used to read NAND
flash memory may cause bit-flips on the surrounding cells and result in
uncorrectable ECC errors. This is known as the read disturb or data
retention. In order to prevent data loss due to the above, all PEBs should
be scrubbed periodically.
In order to solve the problem above, using the 'scan all' sysfs, all the
device PEBs are scheduled for erasure if free, and scheduled for scrubbing
otherwise. To make sure all the PEBs are erased/scrubbed after reset, the
fastmap is flushed at the end.

Change-Id: I0c38907f7086195d64c3f5e5d867b1c965a859c9
Signed-off-by: Dolev Raviv <draviv@codeaurora.org>