uprobes: Fix the wrong usage of current->utask in uprobe_copy_process()

Commit aa59c53fd459 "uprobes: Change uprobe_copy_process() to dup
xol_area" has a stupid typo, we need to setup t->utask->vaddr but
the code wrongly uses current->utask.

Even with this bug dup_xol_work() works "in practice", but only
because get_unmapped_area(NULL, TASK_SIZE - PAGE_SIZE) likely
returns the same address every time.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>

kernel/events/uprobes.c

@@ -1447,7 +1447,7 @@ void uprobe_copy_process(struct task_struct *t, unsigned long flags)
 	if (!work)
 		return uprobe_warn(t, "dup xol area");
 
-	utask->vaddr = area->vaddr;
+	t->utask->vaddr = area->vaddr;
 	init_task_work(work, dup_xol_work);
 	task_work_add(t, work, true);
 }