ubi: eba: properly rollback inside self_check_eba

In case of a memory allocation failure in the volumes loop we can only
process the already allocated scan_eba and fm_eba array elements on the
error path - others are still uninitialized.

Found by Linux Verification Center (linuxtesting.org).

Fixes: 00abf304 ("UBI: Add self_check_eba()")
Cc: stable@vger.kernel.org
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>