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>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/time/timekeeping.c

@@ -166,6 +166,19 @@ static struct timespec total_sleep_time;
  */
 static 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, &wall_to_monotonic,
+			 timekeeper.clock, timekeeper.mult);
+}
+
+
+
 /* flag for if timekeeping is suspended */
 int __read_mostly timekeeping_suspended;
 
@@ -366,11 +379,7 @@ int do_settimeofday(const struct timespec *tv)
 
 	xtime = *tv;
 
-	timekeeper.ntp_error = 0;
-	ntp_clear();
-
-	update_vsyscall(&xtime, &wall_to_monotonic, timekeeper.clock,
-				timekeeper.mult);
+	timekeeping_update(true);
 
 	write_sequnlock_irqrestore(&xtime_lock, flags);
 
@@ -403,11 +412,7 @@ int timekeeping_inject_offset(struct timespec *ts)
 	xtime = timespec_add(xtime, *ts);
 	wall_to_monotonic = timespec_sub(wall_to_monotonic, *ts);
 
-	timekeeper.ntp_error = 0;
-	ntp_clear();
-
-	update_vsyscall(&xtime, &wall_to_monotonic, timekeeper.clock,
-				timekeeper.mult);
+	timekeeping_update(true);
 
 	write_sequnlock_irqrestore(&xtime_lock, flags);
 
@@ -630,10 +635,7 @@ void timekeeping_inject_sleeptime(struct timespec *delta)
 
 	__timekeeping_inject_sleeptime(delta);
 
-	timekeeper.ntp_error = 0;
-	ntp_clear();
-	update_vsyscall(&xtime, &wall_to_monotonic, timekeeper.clock,
-				timekeeper.mult);
+	timekeeping_update(true);
 
 	write_sequnlock_irqrestore(&xtime_lock, flags);
 
@@ -938,9 +940,7 @@ static void update_wall_time(void)
 		wall_to_monotonic.tv_sec -= leap;
 	}
 
-	/* check to see if there is a new clocksource to use */
-	update_vsyscall(&xtime, &wall_to_monotonic, timekeeper.clock,
-				timekeeper.mult);
+	timekeeping_update(false);
 }
 
 /**