timekeeping: Provide internal function __ktime_get_real_seconds

BugLink: http://bugs.launchpad.net/bugs/1519625

In order to fix Y2038 issues in the ntp code we will need replace
get_seconds() with ktime_get_real_seconds() but as the ntp code uses
the timekeeping lock which is also used by ktime_get_real_seconds(),
we need a version without locking.
Add a new function __ktime_get_real_seconds() in timekeeping to
do this.
Reviewed-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: DengChao <chao.deng@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
(cherry picked from commit dee36654)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

kernel/time/timekeeping.c

@@ -845,6 +845,19 @@ time64_t ktime_get_real_seconds(void)
 }
 EXPORT_SYMBOL_GPL(ktime_get_real_seconds);
 
+/**
+ * __ktime_get_real_seconds - The same as ktime_get_real_seconds
+ * but without the sequence counter protect. This internal function
+ * is called just when timekeeping lock is already held.
+ */
+time64_t __ktime_get_real_seconds(void)
+{
+	struct timekeeper *tk = &tk_core.timekeeper;
+
+	return tk->xtime_sec;
+}
+
+
 #ifdef CONFIG_NTP_PPS
 
 /**

kernel/time/timekeeping_internal.h

@@ -26,4 +26,6 @@ static inline cycle_t clocksource_delta(cycle_t now, cycle_t last, cycle_t mask)
 }
 #endif
 
+extern time64_t __ktime_get_real_seconds(void);
+
 #endif /* _TIMEKEEPING_INTERNAL_H */