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/Devices/NullDevice.cpp

@@ -15,6 +15,7 @@ static Singleton<NullDevice> s_the;
 UNMAP_AFTER_INIT void NullDevice::initialize()
 {
     s_the.ensure_instance();
+    s_the->after_inserting();
 }
 
 NullDevice& NullDevice::the()