ubi: fastmap: erase_block: Get erase counter from wl_entry rather than flash

mainline inclusion
from mainline-v6.7-rc1
commit 08a4267874164b2e9c8c50831acd466f47208acc
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=08a4267874164b2e9c8c50831acd466f47208acc



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Just like sync_erase() does, getting erase counter from wl_entry is
faster than reading from flash.

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