locking/rtmutex: replace top-waiter and pi_waiters leftmost caching

... with the generic rbtree flavor instead. No changes
in semantics whatsoever.

Link: http://lkml.kernel.org/r/20170719014603.19029-10-dave@stgolabs.net
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/kernel/fork.c b/kernel/fork.c
index 2ccbbbf..6f1b0af 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1462,8 +1462,7 @@
 {
 	raw_spin_lock_init(&p->pi_lock);
 #ifdef CONFIG_RT_MUTEXES
-	p->pi_waiters = RB_ROOT;
-	p->pi_waiters_leftmost = NULL;
+	p->pi_waiters = RB_ROOT_CACHED;
 	p->pi_top_task = NULL;
 	p->pi_blocked_on = NULL;
 #endif