cpuidle: tracing: Warn about !rcu_is_watching()

When using noinstr, WARN when tracing hits when RCU is disabled.

Suggested-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20230126151323.466670589@infradead.org