btrfs: add comment for optimization in free_extent_buffer()

There's this special atomic compare and exchange logic which serves to
avoid locking the extent buffers refs_lock spinlock and therefore reduce
lock contention, so add a comment to make it more obvious.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: fdmanana

fs/btrfs/extent_io.c

@@ -3494,6 +3494,7 @@ void free_extent_buffer(struct extent_buffer *eb)
 			break;
 		}
 
+		/* Optimization to avoid locking eb->refs_lock. */
 		if (atomic_try_cmpxchg(&eb->refs, &refs, refs - 1))
 			return;
 	}