Revert "Kernel/Storage: Remove the ramdisk implementation"

This reverts commit 4e0f85432a as the
ramdisk code is useful for the bring-up of the aarch64 port. Once the
kernel supports better ram-based filesystems, this code will be removed
again.

Kernel/Storage/Ramdisk/Device.h

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2021, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <Kernel/Locking/Mutex.h>
+#include <Kernel/Storage/StorageDevice.h>
+
+namespace Kernel {
+
+class RamdiskController;
+
+class RamdiskDevice final : public StorageDevice {
+    friend class RamdiskController;
+    friend class DeviceManagement;
+
+public:
+    static NonnullLockRefPtr<RamdiskDevice> create(RamdiskController const&, NonnullOwnPtr<Memory::Region>&& region, int major, int minor);
+    virtual ~RamdiskDevice() override;
+
+    // ^DiskDevice
+    virtual StringView class_name() const override;
+
+private:
+    RamdiskDevice(RamdiskController const&, NonnullOwnPtr<Memory::Region>&&, int major, int minor);
+
+    // ^BlockDevice
+    virtual void start_request(AsyncBlockDeviceRequest&) override;
+
+    // ^StorageDevice
+    virtual CommandSet command_set() const override { return CommandSet::PlainMemory; }
+
+    Mutex m_lock { "RamdiskDevice"sv };
+
+    NonnullOwnPtr<Memory::Region> m_region;
+};
+
+}