UBI: Fastmap: Fix leb_count unbalance

If a LEB is unmapped we have to decrement leb_count as well.

Signed-off-by: Richard Weinberger <richard@nod.at>
Git-commit: ad3d6a05ee45eebf68ff08da0d3f86251b530a27
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


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