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

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>