Kernel+AK: Add and use Userspace<T>::unsafe_userspace_ptr()

Since we already have the type information in the Userspace template,
it was a bit silly to cast manually everywhere. Just add a sufficiently
scary-sounding getter for a typed pointer.

Thanks @alimpfard for pointing out that I was being silly with tossing
out the type.

In the future we may want to make this API non-public as well.

AK/Userspace.h

@@ -45,6 +45,7 @@ public:
     }
 
     FlatPtr ptr() const { return m_ptr; }
+    T unsafe_userspace_ptr() const { return (T)m_ptr; }
 #else
     Userspace(T ptr)
         : m_ptr(ptr)