Usually Fastmap is free to consider every PEB in one of the pools
as newer than the existing PEB. Since PEBs in a pool are by definition
newer than everything else.
But update_vol() missed the case that a pool can contain more than
one candidate.
Cc: <stable@vger.kernel.org> Fixes: dbb7d2a88d ("UBI: Add fastmap core") Signed-off-by: Richard Weinberger <richard@nod.at> Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com> Signed-off-by: Richard Weinberger <richard@nod.at>