[S390] extend virtual timer interface by mod_virt_timer_periodic

In case mod_virt_timer is used to add a non pending timer the timer
is always added as a one-shot timer. If mod_virt_timer is used for
periodic timers they may therfore be degraded to one-shot timers.

Add mod_virt_timer_periodic to the interface to allow safe re-programming
of the interval value.

Signed-off-by: Jan Glauber <jang@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
diff --git a/arch/s390/include/asm/timer.h b/arch/s390/include/asm/timer.h
index e4bcab7..814243c 100644
--- a/arch/s390/include/asm/timer.h
+++ b/arch/s390/include/asm/timer.h
@@ -41,6 +41,7 @@
 extern void add_virt_timer(void *new);
 extern void add_virt_timer_periodic(void *new);
 extern int mod_virt_timer(struct vtimer_list *timer, __u64 expires);
+extern int mod_virt_timer_periodic(struct vtimer_list *timer, __u64 expires);
 extern int del_virt_timer(struct vtimer_list *timer);
 
 extern void init_cpu_vtimer(void);