ubi: fastmap: Wait until there are enough free PEBs before filling pools

Wait until there are enough free PEBs before filling pool/wl_pool,
sometimes erase_worker is not scheduled in time, which causes two
situations:
 A. There are few PEBs filled in pool, which makes ubi_update_fastmap
    is frequently called and leads first 64 PEBs are erased more times
    than other PEBs. So waiting free PEBs before filling pool reduces
    fastmap updating frequency and prolongs flash service life.
 B. In situation that space is nearly running out, ubi_refill_pools()
    cannot make sure pool and wl_pool are filled with free PEBs, caused
    by the delay of erase_worker. After this patch applied, there must
    exist free PEBs in pool after one call of ubi_update_fastmap.

Besides, this patch is a preparetion for fixing large erase counter in
fastmap data block and fixing lapsed wear leveling for first 64 PEBs.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=217787


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