time: Avoid potential shift overflow with large shift values

Andreas Schwab noticed that the 1 << tk->shift could overflow if the
shift value was greater than 30, since 1 would be a 32bit long on
32bit architectures. This issue was introduced by 1e75fa8b (time:
Condense timekeeper.xtime into xtime_sec)

Use 1ULL instead to ensure we don't overflow on the shift.
Reported-by: NAndreas Schwab <schwab@linux-m68k.org>
Signed-off-by: NJohn Stultz <john.stultz@linaro.org>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Link: http://lkml.kernel.org/r/1345595449-34965-4-git-send-email-john.stultz@linaro.orgSigned-off-by: NThomas Gleixner <tglx@linutronix.de>

kernel/time/timekeeping.c

@@ -1184,9 +1184,9 @@ static void update_wall_time(void)
 	* the vsyscall implementations are converted to use xtime_nsec
 	* (shifted nanoseconds), this can be killed.
 	*/
-	remainder = tk->xtime_nsec & ((1 << tk->shift) - 1);
+	remainder = tk->xtime_nsec & ((1ULL << tk->shift) - 1);
 	tk->xtime_nsec -= remainder;
-	tk->xtime_nsec += 1 << tk->shift;
+	tk->xtime_nsec += 1ULL << tk->shift;
 	tk->ntp_error += remainder << tk->ntp_error_shift;
 
 	/*