UBI: fix ubi_wl_flush

The flush function should finish all the pending jobs. But if
somebody else is doing a work, this function should wait and let
it finish.

This patche uses rw semaphore for synchronization purpose - it
just looks quite convinient.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>