ubi: fastmap: Use free pebs reserved for bad block handling

If new bad PEBs occur, UBI firstly consumes ubi->beb_rsvd_pebs, and then
ubi->avail_pebs, finally UBI becomes read-only if above two items are 0,
which means that the amount of PEBs for user volumes is not effected.
Besides, UBI reserves count of free PBEs is ubi->beb_rsvd_pebs while
filling wl pool or getting free PEBs, but ubi->avail_pebs is not reserved.
So ubi->beb_rsvd_pebs and ubi->avail_pebs have nothing to do with the
usage of free PEBs, UBI can use all free PEBs.

Commit 78d6d497 ("UBI: Move fastmap specific functions out of wl.c")
has removed beb_rsvd_pebs checking while filling pool. Now, don't reserve
ubi->beb_rsvd_pebs while filling wl_pool. This will fill more PEBs in pool
and also reduce fastmap updating frequency.

Also remove beb_rsvd_pebs checking in ubi_wl_get_fm_peb.

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>