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>

diff --cc drivers/mtd/ubi/debug.c
Simple merge

diff --cc drivers/mtd/ubi/debug.h
Simple merge

diff --cc drivers/mtd/ubi/ubi.h
Simple merge