ubi: fastmap: Free unused fastmap anchor peb during detach

When CONFIG_MTD_UBI_FASTMAP is enabled, fm_anchor will be assigned
a free PEB during ubi_wl_init() or ubi_update_fastmap(). However
if fastmap is not used or disabled on the MTD device, ubi_wl_entry
related with the PEB will not be freed during detach.

So Fix it by freeing the unused fastmap anchor during detach.

Fixes: f9c34bb5 ("ubi: Fix producing anchor PEBs")
Reported-by:  <syzbot+f317896aae32eb281a58@syzkaller.appspotmail.com>
Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>