- btrfs_path_lock_waiting() looks rather dubious and there's no
  spin_is_contended() method on PREEMPT_RT - so exclude this for now

=> needs a proper fix later. Either this code gets zapped from btrfs
   upstream, or we add spin_is_contended() to PREEMPT_RT too.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>