UBI: Fastmap: Enhance fastmap checking

Don't update the fastmap upon detach if fastmap checking is enabled.
This is poor men's power cut testing feature. :-)

Signed-off-by: Richard Weinberger <richard@nod.at>
Git-commit: 24b7a347c37f99c9d08d2d3ae9c6a56a8333429b
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


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