UBI: Add accessor functions for WL data structures

Fastmap need access to various WL data structures as
fastmap tightly depends on WL.
To make the access less invasive add accessor functions.

Signed-off-by: Richard Weinberger <richard@nod.at>
Git-commit: 23c482ef7b7611225f70efe46a3627b889cab1a4
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


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