fs: btrfs locking workaround for preempt -rt

- 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>

fs/btrfs/locking.c

@@ -94,6 +94,7 @@ static int btrfs_spin_on_block(struct extent_buffer *eb)
  */
 int btrfs_try_spin_lock(struct extent_buffer *eb)
 {
+#ifndef CONFIG_PREEMPT_RT
 	int i;
 
 	if (btrfs_spin_on_block(eb)) {
@@ -113,6 +114,7 @@ int btrfs_try_spin_lock(struct extent_buffer *eb)
 			return 1;
 		spin_unlock(&eb->lock);
 	}
+#endif
 	return 0;
 }