ntp: Access tick_length variable via ntp_tick_length()

Currently the NTP managed tick_length value is accessed globally,
in preparations for locking cleanups, make sure it is accessed via
a function and mark it as static.

CC: Thomas Gleixner <tglx@linutronix.de>
CC: Eric Dumazet <eric.dumazet@gmail.com>
CC: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>

kernel/time/ntp.c

@@ -28,7 +28,7 @@ unsigned long			tick_usec = TICK_USEC;
 /* ACTHZ period (nsecs): */
 unsigned long			tick_nsec;
 
-u64				tick_length;
+static u64			tick_length;
 static u64			tick_length_base;
 
 static struct hrtimer		leap_timer;
@@ -360,6 +360,13 @@ void ntp_clear(void)
 	pps_clear();
 }
 
+
+u64 ntp_tick_length(void)
+{
+	return tick_length;
+}
+
+
 /*
  * Leap second processing. If in leap-insert state at the end of the
  * day, the system clock is set back one second; if in leap-delete