kernel: timer: update k_timer API for const correctness

API that takes k_timer structures but doesn't change data in them is
updated to const-qualify the underlying object, allowing information
to be retrieved from contexts where the containing object is
immutable.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>

include/kernel.h

@@ -2024,9 +2024,9 @@ static inline void z_impl_k_timer_user_data_set(struct k_timer *timer,
  *
  * @return The user data.
  */
-__syscall void *k_timer_user_data_get(struct k_timer *timer);
+__syscall void *k_timer_user_data_get(const struct k_timer *timer);
 
-static inline void *z_impl_k_timer_user_data_get(struct k_timer *timer)
+static inline void *z_impl_k_timer_user_data_get(const struct k_timer *timer)
 {
 	return timer->user_data;
 }

kernel/timer.c

@@ -256,7 +256,7 @@ static inline k_ticks_t z_vrfy_k_timer_expires_ticks(struct k_timer *timer)
 }
 #include <syscalls/k_timer_expires_ticks_mrsh.c>
 
-static inline void *z_vrfy_k_timer_user_data_get(struct k_timer *timer)
+static inline void *z_vrfy_k_timer_user_data_get(const struct k_timer *timer)
 {
 	Z_OOPS(Z_SYSCALL_OBJ(timer, K_OBJ_TIMER));
 	return z_impl_k_timer_user_data_get(timer);