Kernel/Devices: Defer creation of SysFS component after the constructor

Instead of doing so in the constructor, let's do immediately after the
constructor, so we can safely pass a reference of a Device, so the
SysFSDeviceComponent constructor can use that object to identify whether
it's a block device or a character device.
This allows to us to not hold a device in SysFSDeviceComponent with a
RefPtr.
Also, we also call the before_removing method in both SlavePTY::unref
and File::unref, so because Device has that method being overrided, it
can ensure the device is removed always cleanly.

Kernel/FileSystem/File.h

@@ -75,6 +75,7 @@ class File
     , public Weakable<File> {
 public:
     virtual bool unref() const;
+    virtual void before_removing() { }
     virtual ~File();
 
     virtual KResultOr<NonnullRefPtr<OpenFileDescription>> open(int options);