machined: if /var/lib/machines is backed by a loop file, resize it on "machinectl set-limit"

When the pool size limit is altered with "machinectl set-limit", then
not only set the subvolume quota of the /var/lib/machine subvolume, but
also resize the backing loop file and the btrfs file system on it
dynamically.

src/machine/machined-dbus.c

@@ -799,6 +799,10 @@ static int method_set_pool_limit(sd_bus *bus, sd_bus_message *message, void *use
         if (r == 0)
                 return 1; /* Will call us back */
 
+        r = btrfs_resize_loopback("/var/lib/machines", limit);
+        if (r < 0 && r != -ENODEV)
+                return sd_bus_error_set_errnof(error, r, "Failed to adjust loopback limit: %m");
+
         r = btrfs_quota_limit("/var/lib/machines", limit);
         if (r == -ENOTTY)
                 return sd_bus_error_setf(error, SD_BUS_ERROR_NOT_SUPPORTED, "Quota is only supported on btrfs.");