time: Expose get_monotonic_boottime64 for in-kernel use

As part of the 2038 conversion process, add a
get_monotonic_boottime64 accessor so we can depracate
get_monotonic_boottime.

Cc: pang.xunlei <pang.xunlei@linaro.org>
Cc: Arnd Bergmann <arnd.bergmann@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: NJohn Stultz <john.stultz@linaro.org>

include/linux/timekeeping.h

@@ -229,6 +229,11 @@ static inline void get_monotonic_boottime(struct timespec *ts)
 	*ts = ktime_to_timespec(ktime_get_boottime());
 }
 
+static inline void get_monotonic_boottime64(struct timespec64 *ts)
+{
+	*ts = ktime_to_timespec64(ktime_get_boottime());
+}
+
 static inline void timekeeping_clocktai(struct timespec *ts)
 {
 	*ts = ktime_to_timespec(ktime_get_clocktai());