UBI: Fastmap: Add self check to detect absent PEBs

This self check allows Fastmap to detect absent PEBs while
writing a new fastmap to the MTD device.
It will help to find implementation issues in Fastmap.

Signed-off-by: Richard Weinberger <richard@nod.at>
Git-commit: daef3dd1f0ae71bf04a32af4ef643baf040bb73d
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Nikhilesh Reddy <reddyn@codeaurora.org>