UBI: Fix invalidate_fastmap()

Onging tests uncovered that invalidate_fastmap() is broken.
It must not call ubi_wl_put_fm_peb() because all PEBs used
by the old fastmap have already been put back.

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