UBI: Fastmap: Don't allocate new ubi_wl_entry objects

There is no need to allocate new ones every time, we can reuse
the existing ones.
This makes the code cleaner and more easy to follow.

Signed-off-by: Richard Weinberger <richard@nod.at>
Reviewed-by: Tanya Brokhman <tlinder@codeaurora.org>
Reviewed-by: Guido Martínez <guido@vanguardiasur.com.ar>