Btrfs: don't BUG_ON() if we get an error walking backrefs

We can just return false for this so we stop doing the snapshot aware defrag
stuff.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/inode.c

@@ -2129,7 +2129,8 @@ static noinline bool record_extent_backrefs(struct btrfs_path *path,
 						  old->extent_offset, fs_info,
 						  path, record_one_backref,
 						  old);
-		BUG_ON(ret < 0 && ret != -ENOENT);
+		if (ret < 0 && ret != -ENOENT)
+			return false;
 
 		/* no backref to be processed for this extent */
 		if (!old->count) {