pstree: Update max_pid with thread ids

When creating helpers, we need to make sure their
PIDs do not intersect with threads' IDs.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

pstree.c

@@ -415,6 +415,7 @@ static int read_pstree_image(void)
 		for (i = 0; i < e->n_threads; i++) {
 			pi->threads[i].real = -1;
 			pi->threads[i].virt = e->threads[i];
+			max_pid = max((int)e->threads[i], max_pid);
 		}
 
 		task_entries->nr_threads += e->n_threads;