mtd: ubi: wl: Fix error return code in ubi_wl_init()

Fix to return error code -ENOMEM from the kmem_cache_alloc() error
handling case instead of 0, as done elsewhere in this function.

Fixes: f78e5623 ("ubi: fastmap: Erase outdated anchor PEBs during
attach")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Richard Weinberger <richard@nod.at>