btrfs-progs: subvol set-default: change id to 5 if specified as 0

The id 0 of the default subvolume is an internal alias for the toplevel
fs tree, kernel does that conversion. Until 2116398b1d ("btrfs-progs:
use libbtrfsutil for set-default") there was no manual conversion and
the value was passed to kernel as-is. With the switch to the
libbtrfsutil API this got broken (4.19).

  $ btrfs subvol set-default 0 /path

In this case the default subvolume would be containing subvolume of
/path instead of the toplevel one.

Fix it by manually switching the 0 to 5 in case user specifies that to
avoid the difference in the API, that we can't change.

Issue: #327
Reported-by: Chris Murphy
Signed-off-by: David Sterba <dsterba@suse.com>

cmds/subvolume.c

@@ -947,6 +947,13 @@ static int cmd_subvol_set_default(const struct cmd_struct *cmd,
 	} else {
 		/* subvol id and path to the filesystem are specified */
 		objectid = arg_strtou64(argv[optind]);
+		/*
+		 * To avoid confusion with the case above inside libbtrfsutil,
+		 * we must set the toplevel as default manually, same what
+		 * would kernel do.
+		 */
+		if (objectid == 0)
+			objectid = BTRFS_FS_TREE_OBJECTID;
 		path = argv[optind + 1];
 	}