This is the part 1 to fix cyclically reusing single fastmap data PEBs.

After running fsstress on UBIFS for a while, UBI (16384 blocks, fastmap
takes 2 blocks) has an erase block(PEB: 8031) with big erase counter
greater than any other pebs:

=========================================================
from              to     count      min      avg      max
---------------------------------------------------------
0        ..        9:        0        0        0        0
10       ..       99:      532       84       92       99
100      ..      999:    15787      100      147      229
1000     ..     9999:       64     4699     4765     4826
10000    ..    99999:        0        0        0        0
100000   ..      inf:        1   272935   272935   272935
---------------------------------------------------------
Total               :    16384       84      180   272935

Not like fm_anchor, there is no candidate PEBs for fastmap data area,
so old fastmap data pebs will be reused after all free pebs are filled
into pool/wl_pool:
ubi_update_fastmap
 for (i = 1; i < new_fm->used_blocks; i++)
  erase_block(ubi, old_fm->e[i]->pnum)
  new_fm->e[i] = old_fm->e[i]

According to wear leveling algorithm, UBI selects one small erase
counter PEB from ubi->used and one big erase counter PEB from wl_pool,
the reused fastmap data PEB is not in these trees. UBI won't schedule
this PEB for wl even it is in ubi->used because wl algorithm expects
small erase counter for used PEB.

Don't reserve PEB for fastmap in may_reserve_for_fm() if fm_anchor
already exists. Otherwise, when UBI is running out of free PEBs,
the only one free PEB (pnum < 64) will be skipped and fastmap data
will be written on the same old PEB.

Fixes: dbb7d2a8 ("UBI: Add fastmap core")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=217787Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>