ubi: fastmap: Fix missed fm_anchor PEB in wear-leveling after disabling fastmap

After disabling fastmap(ubi->fm_disabled = 1), fastmap won't be updated,
fm_anchor PEB is missed being scheduled for erasing. Besides, fm_anchor
PEB may have smallest erase count, it doesn't participate wear-leveling.
The difference of erase count between fm_anchor PEB and other PEBs will
be larger and larger later on.

In which situation fastmap can be disabled? Initially, we have an UBI
image with fastmap. Then the image will be atttached without module
parameter 'fm_autoconvert', ubi turns to full scanning mode in one
random attaching process(eg. bad fastmap caused by powercut), ubi
fastmap is disabled since then.

Fix it by not getting fm_anchor if fastmap is disabled in
ubi_refill_pools().

Fetch a reproducer in [Link].

Link: https://bugzilla.kernel.org/show_bug.cgi?id=216341


Fixes: 4b68bf9a ("ubi: Select fastmap anchor PEBs considering ...")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>