UBI: prepare for protection tree improvements

This patch modifies @struct ubi_wl_entry and adds union which
contains only one element so far. This is just a preparation
for further changes which will kill the protection tree and
make UBI use a list instead.

Signed-off-by: Xiaochuan-Xu <xiaochuan-xu@cqu.edu.cn>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
diff --git a/drivers/mtd/ubi/ubi.h b/drivers/mtd/ubi/ubi.h
index 1c3fa18..46a4763 100644
--- a/drivers/mtd/ubi/ubi.h
+++ b/drivers/mtd/ubi/ubi.h
@@ -95,7 +95,7 @@
 
 /**
  * struct ubi_wl_entry - wear-leveling entry.
- * @rb: link in the corresponding RB-tree
+ * @u.rb: link in the corresponding (free/used) RB-tree
  * @ec: erase counter
  * @pnum: physical eraseblock number
  *
@@ -104,7 +104,9 @@
  * RB-trees. See WL sub-system for details.
  */
 struct ubi_wl_entry {
-	struct rb_node rb;
+	union {
+		struct rb_node rb;
+	} u;
 	int ec;
 	int pnum;
 };