fix(btrfs): subvolume mount behaviour

Since subvolumes appear automatically when their parent is mounted, no
action is required until the user wants them to be mounted somewhere
else.
This allows access to the volume manager capabilities of BTRFS.

lib/types/btrfs.nix

@@ -91,16 +91,10 @@
         let
           subvolMounts = lib.concatMapAttrs
             (_: subvol:
-              let
-                mountpoint =
-                  if (subvol.mountpoint != null) then subvol.mountpoint
-                  else if (config.mountpoint == null) then subvol.name
-                  else null;
-              in
-              lib.optionalAttrs (mountpoint != null) {
-                ${mountpoint} = ''
-                  if ! findmnt ${config.device} "${rootMountPoint}${mountpoint}" > /dev/null 2>&1; then
-                    mount ${config.device} "${rootMountPoint}${mountpoint}" \
+              lib.optionalAttrs (subvol.mountpoint != null) {
+                ${subvol.mountpoint} = ''
+                  if ! findmnt ${config.device} "${rootMountPoint}${subvol.mountpoint}" > /dev/null 2>&1; then
+                    mount ${config.device} "${rootMountPoint}${subvol.mountpoint}" \
                     ${lib.concatMapStringsSep " " (opt: "-o ${opt}") (subvol.mountOptions ++ [ "subvol=${subvol.name}" ])} \
                     -o X-mount.mkdir
                   fi