f2fs: clean up structure extent_node

The union in struct extent_node wass only to indicate below fields

	struct rb_node rb_node;
	union {
		struct {
			unsigned int fofs;
			unsigned int len;
		...
	...

can be parsed as fields in struct rb_entry, but they were never be
used explicitly before, so let's remove them for cleanup.
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/f2fs.h

@@ -559,16 +559,8 @@ struct extent_info {
 };
 
 struct extent_node {
-	struct rb_node rb_node;
-	union {
-		struct {
-			unsigned int fofs;
-			unsigned int len;
-			u32 blk;
-		};
-		struct extent_info ei;	/* extent info */
-
-	};
+	struct rb_node rb_node;		/* rb node located in rb-tree */
+	struct extent_info ei;		/* extent info */
 	struct list_head list;		/* node in global extent list of sbi */
 	struct extent_tree *et;		/* extent tree pointer */
 };