btrfs-progs: error out immediately if an unknown backref type is found

There is a bug report that for fuzzed image
bko-155621-bad-block-group-offset.raw, "btrfs check --mode=lowmem
--repair" would lead to an endless loop.

Unlike original mode, lowmem mode relies on the backref walk to properly
go through each root, but unfortunately inside __add_inline_refs() we
doesn't handle unknown backref types correctly, causing it never moving
forward thus deadloop.

Fix it by erroring out to prevent an endless loop.

Issue: #788
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

kernel-shared/backref.c

@@ -650,7 +650,8 @@ static int __add_inline_refs(struct btrfs_fs_info *fs_info,
 			break;
 		}
 		default:
-			WARN_ON(1);
+			error("invalid backref type: %u", type);
+			ret = -EUCLEAN;
 		}
 		if (ret)
 			return ret;