UBI: Fix PEB leak in wear_leveling_worker()

get_peb_for_wl() removes the PEB from the free list.
If the WL subsystem detects that no wear leveling is needed
it cancels the operation and drops the gained PEB.
In this case we have to put the PEB back into the free list.

This issue was introduced with commit ed4b7021
(UBI: remove PEB from free tree in get_peb_for_wl()).

Cc: <stable@vger.kernel.org> # 3.7.x
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>