kernel/futex: Kill rt_mutex_next_owner()

Update wake_futex_pi() and kill the call altogether. This is possible because:

(i) The case of fixup_owner() in which the pi_mutex was stolen from the
signaled enqueued top-waiter which fails to trylock and doesn't see a
current owner of the rtmutex but needs to acknowledge an non-enqueued
higher priority waiter, which is the other alternative. This used to be
handled by rt_mutex_next_owner(), which guaranteed fixup_pi_state_owner('newowner')
never to be nil. Nowadays the logic is handled by an EAGAIN loop, without
the need of rt_mutex_next_owner(). Specifically:

    c1e2f0eaf0 (futex: Avoid violating the 10th rule of futex)
    9f5d1c336a (futex: Handle transient "ownerless" rtmutex state correctly)

(ii) rt_mutex_next_owner() and rt_mutex_top_waiter() are semantically
equivalent, as of:

    c28d62cf52 (locking/rtmutex: Handle non enqueued waiters gracefully in remove_waiter())

So instead of keeping the call around, just use the good ole rt_mutex_top_waiter().
No change in semantics.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210226175029.50335-1-dave@stgolabs.net

kernel/locking/rtmutex.c

@@ -1792,26 +1792,6 @@ int rt_mutex_start_proxy_lock(struct rt_mutex *lock,
 	return ret;
 }
 
-/**
- * rt_mutex_next_owner - return the next owner of the lock
- *
- * @lock: the rt lock query
- *
- * Returns the next owner of the lock or NULL
- *
- * Caller has to serialize against other accessors to the lock
- * itself.
- *
- * Special API call for PI-futex support
- */
-struct task_struct *rt_mutex_next_owner(struct rt_mutex *lock)
-{
-	if (!rt_mutex_has_waiters(lock))
-		return NULL;
-
-	return rt_mutex_top_waiter(lock)->task;
-}
-
 /**
  * rt_mutex_wait_proxy_lock() - Wait for lock acquisition
  * @lock:		the rt_mutex we were woken on