idr: reorder the fields

idr_layer->layer is always accessed in read path, move it in the front.

idr_layer->bitmap is moved on the bottom. And rcu_head shares with
bitmap due to they do not be accessed at the same time.

idr->id_free/id_free_cnt/lock are free list fields, and moved to the
bottom. They will be removed in near future.

Signed-off-by: Lai Jiangshan <[e-mail]>
Cc: Tejun Heo <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/idr.h

···
 
 struct idr_layer {
 	int			prefix;	/* the ID prefix of this idr_layer */
-	DECLARE_BITMAP(bitmap, IDR_SIZE); /* A zero bit means "space here" */
+	int			layer;	/* distance from leaf */
 	struct idr_layer __rcu	*ary[1<<IDR_BITS];
 	int			count;	/* When zero, we can release it */
-	int			layer;	/* distance from leaf */
-	struct rcu_head		rcu_head;
+	union {
+		/* A zero bit means "space here" */
+		DECLARE_BITMAP(bitmap, IDR_SIZE);
+		struct rcu_head		rcu_head;
+	};
 };
 
 struct idr {
 	struct idr_layer __rcu	*hint;	/* the last layer allocated from */
 	struct idr_layer __rcu	*top;
-	struct idr_layer	*id_free;
 	int			layers;	/* only valid w/o concurrent changes */
-	int			id_free_cnt;
 	int			cur;	/* current pos for cyclic allocation */
 	spinlock_t		lock;
+	int			id_free_cnt;
+	struct idr_layer	*id_free;
 };
 
 #define IDR_INIT(name)							\