UBI: Fastmap: Rename variables to make them meaningful

s/fmpl1/fmpl
s/fmpl2/fmpl_wl

Add "WL" to the error message when wrong WL pool magic number is detected.

Signed-off-by: Sheng Yong <shengyong1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
Git-commit: f6e951af34c515ffc69f4a557cf62533420c8ce4
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


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