time: Move common updates to a function

This is a backport of cc06268c

While not a bugfix itself, it allows following fixes to backport
in a more straightforward manner.

CC: Thomas Gleixner <tglx@linutronix.de>
CC: Eric Dumazet <eric.dumazet@gmail.com>
CC: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Linux Kernel <linux-kernel@vger.kernel.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>

kernel/time/timekeeping.c

@@ -166,6 +166,18 @@ static struct timespec total_sleep_time;
  */
 struct timespec raw_time;
 
+/* must hold write on xtime_lock */
+static void timekeeping_update(bool clearntp)
+{
+	if (clearntp) {
+		timekeeper.ntp_error = 0;
+		ntp_clear();
+	}
+	update_vsyscall(&xtime, timekeeper.clock, timekeeper.mult);
+}
+
+
+
 /* flag for if timekeeping is suspended */
 int __read_mostly timekeeping_suspended;
 
@@ -341,10 +353,7 @@ int do_settimeofday(struct timespec *tv)
 
 	update_xtime_cache(0);
 
-	timekeeper.ntp_error = 0;
-	ntp_clear();
-
-	update_vsyscall(&xtime, timekeeper.clock, timekeeper.mult);
+	timekeeping_update(true);
 
 	write_sequnlock_irqrestore(&xtime_lock, flags);
 
@@ -832,8 +841,7 @@ void update_wall_time(void)
 	nsecs = clocksource_cyc2ns(offset, timekeeper.mult, timekeeper.shift);
 	update_xtime_cache(nsecs);
 
-	/* check to see if there is a new clocksource to use */
-	update_vsyscall(&xtime, timekeeper.clock, timekeeper.mult);
+	timekeeping_update(false);
 }
 
 /**