unwind: Clarify calling context

The get_cookie() function hard relies on IRQs being disabled, but this
isn't immediately obvious when reading the function.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Link: https://patch.msgid.link/20250924080119.122507632@infradead.org

kernel/unwind/deferred.c

@@ -79,6 +79,8 @@ static u64 get_cookie(struct unwind_task_info *info)
 {
 	u32 cnt = 1;
 
+	lockdep_assert_irqs_disabled();
+
 	if (info->id.cpu)
 		return info->id.id;