ubi: fastmap: Get wl PEB even ec beyonds the 'max' if free PEBs are run out

This is the part 2 to fix cyclically reusing single fastmap data PEBs.

Consider one situation, if there are four free PEBs for fm_anchor, pool,
wl_pool and fastmap data PEB with erase counter 100, 100, 100, 5096
(ubi->beb_rsvd_pebs is 0). PEB with erase counter 5096 is always picked
for fastmap data according to the realization of find_wl_entry(), since
fastmap data PEB is not scheduled for wl, finally there are two PEBs
(fm data) with great erase counter than other PEBS.
Get wl PEB even its erase counter exceeds the 'max' in find_wl_entry()
when free PEBs are run out after filling pools and fm data. Then the PEB
with biggest erase conter is taken as wl PEB, it can be scheduled for wl.

Fixes: dbb7d2a8 ("UBI: Add fastmap core")
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>