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>