UBI: fastmap: use ubi_find_volume() instead of open coding it

process_pool_aeb() re-implements the logic found in ubi_find_volume().
Call ubi_find_volume() to avoid this duplication.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Richard Weinberger <richard@nod.at>