timers: introduce try_to_del_timer_sync()

This patch splits del_timer_sync() into 2 functions. The new one, try_to_del_timer_sync(), returns -1 when it hits executing timer.

It can be used in interrupt context, or when the caller hold locks which can prevent completion of the timer's handler.

NOTE. Currently it can't be used in interrupt context in UP case, because ->running_timer is used only with CONFIG_SMP.

Should the need arise, it is possible to kill #ifdef CONFIG_SMP in set_running_timer(), it is cheap.

@@ -76,9 +76,11 @@ static inline void add_timer(struct time
 }
 
 #ifdef CONFIG_SMP
+  extern int try_to_del_timer_sync(struct timer_list *timer);
   extern int del_timer_sync(struct timer_list *timer);
 #else
-# define del_timer_sync(t) del_timer(t)
+# define try_to_del_timer_sync(t)	del_timer(t)
+# define del_timer_sync(t)		del_timer(t)
 #endif
 
 #define del_singleshot_timer_sync(t) del_timer_sync(t)