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Fixes: f78e5623 ("ubi: fastmap: Erase outdated anchor PEBs during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