ubi: fastmap: Check wl_pool for free peb before wear leveling

mainline inclusion
from mainline-v5.19-rc1
commit 14072ee3
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I9195H
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=14072ee33d5a2a07d735b667205f99d0a95de9f2



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UBI fetches free peb from wl_pool during wear leveling, so UBI should
check wl_pool's empty status before wear leveling. Otherwise, UBI will
miss wear leveling chances when free pebs are run out.

Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>