ubi: fastmap: wl: Schedule fm_work if wear-leveling pool is empty

ubi: fastmap: wl: Schedule fm_work if wear-leveling pool is empty

Since commit 14072ee33d5a ("ubi: fastmap: Check wl_pool for free peb
before wear leveling"), wear_leveling_worker() won't schedule fm_work
if wear-leveling pool is empty, which could temporarily disable the
wear-leveling until the fastmap is updated(eg. pool becomes empty).
Fix it by scheduling fm_work if wl_pool is empty during wear-leveing.

Fixes: 14072ee33d5a ("ubi: fastmap: Check wl_pool for free peb before wear leveling")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>