uretprobes: Return probe entry, prepare_uretprobe()

When a uprobe with return probe consumer is hit, prepare_uretprobe()
function is invoked. It creates return_instance, hijacks return address
and replaces it with the trampoline.

* Return instances are kept as stack per uprobed task.
* Return instance is chained, when the original return address is
  trampoline's page vaddr (e.g. recursive call of the probed function).

Signed-off-by: Anton Arapov <anton@redhat.com>
Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>

include/linux/uprobes.h

@@ -71,6 +71,7 @@ struct uprobe_task {
 	enum uprobe_task_state		state;
 	struct arch_uprobe_task		autask;
 
+	struct return_instance		*return_instances;
 	struct uprobe			*active_uprobe;
 
 	unsigned long			xol_vaddr;