timer/debug: Change /proc/timer_list from 0444 to 0400

While it uses %pK, there's still few reasons to read this file
as non-root.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>