UBI: Fix livelock in produce_free_peb()

The while loop in produce_free_peb() assumes that each work will produce a
free PEB. This is not true.
If ubi->works_count is 1 and the only scheduled work is the
wear_leveling_worker() produce_free_peb() can loop forever in case
nobody schedules an erase work.
Fix this issue by checking in the while loop whether work is scheduled.

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

drivers/mtd/ubi/wl.c

@@ -272,7 +272,7 @@ static int produce_free_peb(struct ubi_device *ubi)
 {
 	int err;
 
-	while (!ubi->free.rb_node) {
+	while (!ubi->free.rb_node && ubi->works_count) {
 		spin_unlock(&ubi->wl_lock);
 
 		dbg_wl("do one work synchronously");