locking/rtmutex: Update stale plist comments

locking/rtmutex: Update stale plist comments

... as of fb00aca4744 (rtmutex: Turn the plist into an rb-tree) we
no longer use plists for queuing any waiters. Update stale comments.

Signed-off-by: Davidlohr Bueso <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Sebastian Andrzej Siewior <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Thomas Gleixner <[email protected]>