UBI: Add initial support for fastmap self checks

Using this debugfs knob fastmap self checks can be controlled.

Signed-off-by: Richard Weinberger <richard@nod.at>
Reviewed-by: Tanya Brokhman <tlinder@codeaurora.org>
Git-commit: 5fa7fa5dad0cf8f2b47bb8dca91c605c48d282f6
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


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