uprobes: Re-order struct uprobe_task to save some space

On x86_64, with allmodconfig, struct uprobe_task is 72 bytes long, with a
hole and some padding.

/* size: 72, cachelines: 2, members: 7 */
/* sum members: 64, holes: 1, sum holes: 4 */
/* padding: 4 */
/* forced alignments: 1, forced holes: 1, sum forced holes: 4 */
/* last cacheline: 8 bytes */

Reorder the structure to fill the hole and avoid the padding.

This way, the whole structure fits in a single cacheline and some memory is
saved when it is allocated.

/* size: 64, cachelines: 1, members: 7 */
/* forced alignments: 1 */

Signed-off-by: Christophe JAILLET <[e-mail]>
Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Acked-by: "Masami Hiramatsu (Google)" <[e-mail]>
Link: https://lore.kernel.org/r/a9f541d0ce[e-mail]

include/linux/uprobes.h

···
 struct uprobe_task {
 	enum uprobe_task_state		state;
 
+	unsigned int			depth;
+	struct return_instance		*return_instances;
+
 	union {
 		struct {
 			struct arch_uprobe_task	autask;
···
 	unsigned long			xol_vaddr;
 
 	struct arch_uprobe              *auprobe;
-
-	struct return_instance		*return_instances;
-	unsigned int			depth;
 };
 
 struct return_consumer {