UBI: Fastmap: Make WL pool size 50% of user pool size

Don't use a fixed size for the WL pool.
Make it instead 50% of the user pool.
We don't make it 100% as it is not as heavily used as the user pool.

Signed-off-by: Richard Weinberger <richard@nod.at>
Git-commit: 68e3226bd4e05c4be1f6286e4e708e1163ffa092
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


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