ubi: Don't do anchor move within fastmap area

To make sure that Fastmap can use a PEB within the first 64
PEBs, UBI moves blocks away from that area.
It uses regular wear-leveling for that job.

An anchor move can be triggered if no PEB is free in this area
or because of anticipation. In the latter case it can happen
that UBI decides to move a block but finds a free PEB
within the same area.
This case is in vain an increases only erase counters.

Catch this case and cancel wear-leveling if this happens.

Signed-off-by: Richard Weinberger <richard@nod.at>