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.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>