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>
diff --git a/drivers/mtd/ubi/ubi.h b/drivers/mtd/ubi/ubi.h
index ea9a699..994233d 100644
--- a/drivers/mtd/ubi/ubi.h
+++ b/drivers/mtd/ubi/ubi.h
@@ -355,6 +355,7 @@
 	} prot;
 	spinlock_t wl_lock;
 	struct mutex move_mutex;
+	struct rw_semaphore work_sem;
 	int wl_scheduled;
 	struct ubi_wl_entry **lookuptbl;
 	unsigned long long abs_ec;