Don't 'printk()' while holding xtime lock for writing

The printk() can deadlock because it can wake up klogd(), and
task enqueueing will try to read the time in order to set a hrtimer.
Reported-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Debugged-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/time/timekeeping.c

@@ -191,8 +191,12 @@ static void change_clocksource(void)
 
 	tick_clock_notify();
 
+	/*
+	 * We're holding xtime lock and waking up klogd would deadlock
+	 * us on enqueue.  So no printing!
 	printk(KERN_INFO "Time: %s clocksource has been installed.\n",
 	       clock->name);
+	 */
 }
 #else
 static inline void change_clocksource(void) { }