Kernel: Make VirtualFileSystem::Mount a top-level class

And move it to its own compilation unit.

Kernel/FileSystem/Forward.h

@@ -0,0 +1,17 @@
+/*
+ * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+namespace Kernel {
+
+class FileSystem;
+class Inode;
+class Mount;
+class VirtualFileSystem;
+struct InodeMetadata;
+
+}