ubi: fastmap: Free fastmap next anchor peb during detach

ubi_wl_entry related with the fm_next_anchor PEB is not freed during
detach, which causes a memory leak.
Don't forget to release fm_next_anchor PEB while detaching ubi from
mtd when CONFIG_MTD_UBI_FASTMAP is enabled.

Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Fixes: 4b68bf9a

 ("ubi: Select fastmap anchor PEBs considering...")
Signed-off-by: Richard Weinberger <richard@nod.at>