UBI: Fastmap: Prepare for variable sized fastmaps

The current code assumes that each fastmap has the same amount of PEBs.
So far this is true but will change soon.

Signed-off-by: Richard Weinberger <richard@nod.at>
Git-commit: 61de74ce2fa2cc9eee85cf5edc6525b12831b507
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


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