Kernel: Utilize AK::Userspace<T> in the ioctl interface

It's easy to forget the responsibility of validating and safely copying
kernel parameters in code that is far away from syscalls. ioctl's are
one such example, and bugs there are just as dangerous as at the root
syscall level.

To avoid this case, utilize the AK::Userspace<T> template in the ioctl
kernel interface so that implementors have no choice but to properly
validate and copy ioctl pointer arguments.

Kernel/FileSystem/File.cpp

@@ -5,6 +5,7 @@
  */
 
 #include <AK/StringView.h>
+#include <AK/Userspace.h>
 #include <Kernel/FileSystem/File.h>
 #include <Kernel/FileSystem/FileDescription.h>
 #include <Kernel/Process.h>
@@ -34,7 +35,7 @@ KResult File::close()
     return KSuccess;
 }
 
-int File::ioctl(FileDescription&, unsigned, FlatPtr)
+int File::ioctl(FileDescription&, unsigned, Userspace<void*>)
 {
     return -ENOTTY;
 }