filesystems: Add autoResize assertion

Assert that autoResize is only used when fsType is explicitly set to a
supported filesystem: if it's set to "auto", the default, the required
resizing tools won't be copied into the initrd even if the actual
filesystem is supported.

nixos/modules/tasks/filesystems.nix

@@ -209,10 +209,17 @@ in
 
     assertions = let
       ls = sep: concatMapStringsSep sep (x: x.mountPoint);
+      notAutoResizable = fs: fs.autoResize && !(hasPrefix "ext" fs.fsType || fs.fsType == "f2fs");
     in [
       { assertion = ! (fileSystems' ? "cycle");
         message = "The ‘fileSystems’ option can't be topologically sorted: mountpoint dependency path ${ls " -> " fileSystems'.cycle} loops to ${ls ", " fileSystems'.loops}";
       }
+      { assertion = ! (any notAutoResizable fileSystems);
+        message = let
+          fs = head (filter notAutoResizable fileSystems);
+        in
+          "Mountpoint '${fs.mountPoint}': 'autoResize = true' is not supported for 'fsType = \"${fs.fsType}\"':${if fs.fsType == "auto" then " fsType has to be explicitly set and" else ""} only the ext filesystems and f2fs support it.";
+      }
     ];
 
     # Export for use in other modules