s390/time: avoid races when updating tb_update_count

The increment might not be atomic and we're not holding the
timekeeper_lock. Therefore we might lose an update to count, resulting in
VDSO being trapped in a loop. As other archs also simply update the
values and count doesn't seem to have an impact on reloading of these
values in VDSO code, let's just remove the update of tb_update_count.
Suggested-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/time.c

@@ -278,13 +278,8 @@ extern struct timezone sys_tz;
 
 void update_vsyscall_tz(void)
 {
-	/* Make userspace gettimeofday spin until we're done. */
-	++vdso_data->tb_update_count;
-	smp_wmb();
 	vdso_data->tz_minuteswest = sys_tz.tz_minuteswest;
 	vdso_data->tz_dsttime = sys_tz.tz_dsttime;
-	smp_wmb();
-	++vdso_data->tb_update_count;
 }
 
 /*