UBI: Fix memory leak in ubi_attach_fastmap() error path

On error we have to free all three temporary lists.

Reported-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>