rt: Make rt_down_read_trylock() behave like down_read_trylock()

This patch removes the stupid "Read locks within the self-held write
lock succeed" behaviour. This is breaking in mm_take_all_locks() since
it is quite common to ensure that a lock is taken with
BUG_ON(down_read_trylock(&mm->mmap_sem)).
Signed-off-by: Jan Blunck <jblunck@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/rt.c

@@ -380,18 +380,6 @@ int  rt_down_read_trylock(struct rw_semaphore *rwsem)
 	unsigned long flags;
 	int ret;
 
-	/*
-	 * Read locks within the self-held write lock succeed.
-	 */
-	spin_lock_irqsave(&rwsem->lock.wait_lock, flags);
-	if (rt_mutex_real_owner(&rwsem->lock) == current) {
-		spin_unlock_irqrestore(&rwsem->lock.wait_lock, flags);
-		rwsem_acquire_read(&rwsem->dep_map, 0, 1, _RET_IP_);
-		rwsem->read_depth++;
-		return 1;
-	}
-	spin_unlock_irqrestore(&rwsem->lock.wait_lock, flags);
-
 	ret = rt_mutex_trylock(&rwsem->lock);
 	if (ret)
 		rwsem_acquire(&rwsem->dep_map, 0, 1, _RET_IP_);