UBI: fix out of bounds write

commit d74adbdb upstream.

If aeb->len >= vol->reserved_pebs, we should not be writing aeb into the
PEB->LEB mapping.

Caught by Coverity, CID #711212.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>