Propagate filesystem mountOptions to subvolumes

Make subvolume mountOptions also include options for the whole
filesystem, but only when the filesystems mountpoint is not specified

Author: VlaDexa

lib/types/bcachefs_filesystem.nix

@@ -319,6 +319,7 @@
                 "X-mount.subdir=${lib.removePrefix "/" subvolume.name}"
               ]
               ++ subvolume.mountOptions
+              ++ lib.optional (config.mountpoint == null) config.mountOptions
             );
             neededForBoot = true;
           };