ubi: fastmap: Erase outdated anchor PEBs during attach

The fastmap update code might erase the current fastmap anchor PEB
in case it doesn't find any new free PEB. When a power cut happens
in this situation we must not have any outdated fastmap anchor PEB
on the device, because that would be used to attach during next
boot.
The easiest way to make that sure is to erase all outdated fastmap
anchor PEBs synchronously during attach.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Richard Weinberger <richard@nod.at>
Fixes: dbb7d2a8 ("UBI: Add fastmap core")
Cc: <stable@vger.kernel.org>
Signed-off-by: Richard Weinberger <richard@nod.at>