locking/rtmutex: Fix misleading comment in rt_mutex_postunlock()

Preemption is disabled in mark_wakeup_next_waiter(,) not in
rt_mutex_slowunlock().

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210326153944.341734608@linutronix.de