s390/time: adjust last_update_clock at clock synchronization

The last_update_clock time stamp in the lowcore should be adjusted by
the TOD clock delta that is created by the clock synchronization.
Otherwise the calculation of the steal time will be incorrect.
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/time.c

@@ -401,6 +401,8 @@ static void clock_sync_local(unsigned long long delta)
 		S390_lowcore.clock_comparator += delta;
 		set_clock_comparator(S390_lowcore.clock_comparator);
 	}
+	/* Adjust the last_update_clock time-stamp. */
+	S390_lowcore.last_update_clock += delta;
 }
 
 /* Single threaded workqueue used for stp sync events */