UBI: Fastmap: Switch to ro mode if invalidate_fastmap() fails

We have to switch to ro mode to guarantee that upon next UBI attach
all data is consistent.

Signed-off-by: Richard Weinberger <richard@nod.at>
Git-commit: 2d93fb3632755c170bc684989709e28ef3d22a80
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


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