posix-timers: Add timer_rearm() callback

Add a timer_rearm() callback which is used to make the rescheduling of
posix interval timers independent of the underlying clock implementation.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: John Stultz <john.stultz@linaro.org>
Link: http://lkml.kernel.org/r/20170530211656.732632167@linutronix.de

kernel/time/posix-timers.h

 #define TIMER_RETRY 1
 
 struct k_clock {
-	int (*clock_getres) (const clockid_t which_clock, struct timespec64 *tp);
-	int (*clock_set) (const clockid_t which_clock,
-			  const struct timespec64 *tp);
-	int (*clock_get) (const clockid_t which_clock, struct timespec64 *tp);
-	int (*clock_adj) (const clockid_t which_clock, struct timex *tx);
-	int (*timer_create) (struct k_itimer *timer);
-	int (*nsleep) (const clockid_t which_clock, int flags,
-		       struct timespec64 *, struct timespec __user *);
-	long (*nsleep_restart) (struct restart_block *restart_block);
-	int (*timer_set) (struct k_itimer *timr, int flags,
-			  struct itimerspec64 *new_setting,
-			  struct itimerspec64 *old_setting);
-	int (*timer_del) (struct k_itimer *timr);
-	void (*timer_get) (struct k_itimer *timr,
-			   struct itimerspec64 *cur_setting);
+	int	(*clock_getres)(const clockid_t which_clock,
+				struct timespec64 *tp);
+	int	(*clock_set)(const clockid_t which_clock,
+			     const struct timespec64 *tp);
+	int	(*clock_get)(const clockid_t which_clock,
+			     struct timespec64 *tp);
+	int	(*clock_adj)(const clockid_t which_clock, struct timex *tx);
+	int	(*timer_create)(struct k_itimer *timer);
+	int	(*nsleep)(const clockid_t which_clock, int flags,
+			  struct timespec64 *, struct timespec __user *);
+	long	(*nsleep_restart)(struct restart_block *restart_block);
+	int	(*timer_set)(struct k_itimer *timr, int flags,
+			     struct itimerspec64 *new_setting,
+			     struct itimerspec64 *old_setting);
+	int	(*timer_del)(struct k_itimer *timr);
+	void	(*timer_get)(struct k_itimer *timr,
+			     struct itimerspec64 *cur_setting);
+	void	(*timer_rearm)(struct k_itimer *timr);
 };
 
 extern const struct k_clock clock_posix_cpu;