ubi: fastmap: Check each mapping only once

commit 34653fd8c46e771585fce5975e4243f8fd401914 upstream.

Maintain a bitmap to keep track of which LEB->PEB mapping
was checked already.
That way we have to read back VID headers only once.

Fixes: a23cf10d ("ubi: fastmap: Correctly handle interrupted erasures in EBA")
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Martin Kepplinger <martin.kepplinger@ginzinger.com>