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/TTY/PTYMultiplexer.cpp

@@ -35,6 +35,11 @@ UNMAP_AFTER_INIT PTYMultiplexer::~PTYMultiplexer()
 {
 }
 
+void PTYMultiplexer::initialize()
+{
+    the().after_inserting();
+}
+
 KResultOr<NonnullRefPtr<OpenFileDescription>> PTYMultiplexer::open(int options)
 {
     return m_freelist.with_exclusive([&](auto& freelist) -> KResultOr<NonnullRefPtr<OpenFileDescription>> {