clocksource: Add module refcount

Add a module refcount, so the current clocksource cannot be removed
unconditionally.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Magnus Damm <magnus.damm@gmail.com>
Link: http://lkml.kernel.org/r/20130425143435.762417789@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

include/linux/clocksource.h

@@ -21,6 +21,7 @@
 /* clocksource cycle base type */
 typedef u64 cycle_t;
 struct clocksource;
+struct module;
 
 #ifdef CONFIG_ARCH_CLOCKSOURCE_DATA
 #include <asm/clocksource.h>
@@ -162,6 +163,7 @@ extern u64 timecounter_cyc2time(struct timecounter *tc,
  * @suspend:		suspend function for the clocksource, if necessary
  * @resume:		resume function for the clocksource, if necessary
  * @cycle_last:		most recent cycle counter value seen by ::read()
+ * @owner:		module reference, must be set by clocksource in modules
  */
 struct clocksource {
 	/*
@@ -195,6 +197,7 @@ struct clocksource {
 	cycle_t cs_last;
 	cycle_t wd_last;
 #endif
+	struct module *owner;
 } ____cacheline_aligned;
 
 /*