ubi: Initialize Fastmap checkmapping correctly

commit 25677478474a91fa1b46f19a4a591a9848bca6fb upstream

We cannot do it last, otherwithse it will be skipped for dynamic
volumes.

Reported-by: Lachmann, Juergen <juergen.lachmann@harman.com>
Fixes: 34653fd8c46e ("ubi: fastmap: Check each mapping only once")
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>