locking/rtmutex: Don't initialize lockdep when not required

pi_mutex isn't supposed to be tracked by lockdep, but just
passing NULLs for name and key will cause lockdep to spew a
warning and die, which is not what we want it to do.

Skip lockdep initialization if the caller passed NULLs for
name and key, suggesting such initialization isn't desired.
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: f5694788 ("rt_mutex: Add lockdep annotations")
Link: http://lkml.kernel.org/r/20170618140548.4763-1-alexander.levin@verizon.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

kernel/locking/rtmutex.c

@@ -1661,6 +1661,7 @@ void __rt_mutex_init(struct rt_mutex *lock, const char *name,
 	lock->waiters = RB_ROOT;
 	lock->waiters_leftmost = NULL;
 
+	if (name && key)
 		debug_rt_mutex_init(lock, name, key);
 }
 EXPORT_SYMBOL_GPL(__rt_mutex_init);