pstree: Link pid and pstree_item

Add a link from task's pid to pstree_item.
Threads have this link set in NULL.

travis-ci: success for Make pstree_item::pid allocated dynamically
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/include/pid.h

@@ -5,6 +5,7 @@
 #include "rbtree.h"
 
 struct pid {
+	struct pstree_item *item;
 	/*
 	 * The @real pid is used to fetch tasks during dumping stage,
 	 * This is a global pid seen from the context where the dumping

criu/pstree.c

@@ -216,6 +216,7 @@ struct pstree_item *__alloc_pstree_item(bool rst)
 	item->pid.virt = -1;
 	item->pid.real = -1;
 	item->born_sid = -1;
+	item->pid.item = item;
 
 	return item;
 }
@@ -554,6 +555,7 @@ static int read_pstree_image(pid_t *pid_max)
 			pi->threads[i].real = -1;
 			pi->threads[i].virt = e->threads[i];
 			pi->threads[i].state = TASK_THREAD;
+			pi->threads[i].item = NULL;
 			if (i == 0)
 				continue; /* A thread leader is in a tree already */
 			node = lookup_create_pid(pi->threads[i].virt, &pi->threads[i]);

criu/seize.c

@@ -695,6 +695,7 @@ static int collect_threads(struct pstree_item *item)
 	if (item->nr_threads == 0) {
 		item->threads[0].real = item->pid.real;
 		item->nr_threads = 1;
+		item->threads[0].item = NULL;
 	}
 
 	nr_inprogress = 0;
@@ -732,6 +733,7 @@ static int collect_threads(struct pstree_item *item)
 
 		BUG_ON(item->nr_threads + 1 > nr_threads);
 		item->threads[item->nr_threads].real = pid;
+		item->threads[item->nr_threads].item = NULL;
 		item->nr_threads++;
 
 		if (ret == TASK_DEAD) {