posix-timers: Document sys_clock_settime() permissions in place

The documentation of sys_clock_settime() permissions is at a random place
and mostly word salad.

Remove it and add a concise comment into sys_clock_settime().

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20230425183313.514700292@linutronix.de

Author: KAGA-KOKO

kernel/time/posix-timers.c

@@ -74,13 +74,6 @@ static const struct k_clock clock_realtime, clock_monotonic;
  *	    following: 1.) The k_itimer struct (sched.h) is used for
  *	    the timer.  2.) The list, it_lock, it_clock, it_id and
  *	    it_pid fields are not modified by timer code.
- *
- * Permissions: It is assumed that the clock_settime() function defined
- *	    for each clock will take care of permission checks.	 Some
- *	    clocks may be set able by any user (i.e. local process
- *	    clocks) others not.	 Currently the only set able clock we
- *	    have is CLOCK_REALTIME and its high res counter part, both of
- *	    which we beg off on and pass to do_sys_settimeofday().
  */
 static struct k_itimer *__lock_timer(timer_t timer_id, unsigned long *flags);
 
@@ -1159,6 +1152,10 @@ SYSCALL_DEFINE2(clock_settime, const clockid_t, which_clock,
 	if (get_timespec64(&new_tp, tp))
 		return -EFAULT;
 
+	/*
+	 * Permission checks have to be done inside the clock specific
+	 * setter callback.
+	 */
 	return kc->clock_set(which_clock, &new_tp);
 }