ubi: Initialize Fastmap checkmapping correctly

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

Reported-by: Lachmann, Juergen <juergen.lachmann@harman.com>
Fixes: 34653fd8 ("ubi: fastmap: Check each mapping only once")
Signed-off-by: Richard Weinberger <richard@nod.at>