AK: Add static_ptr_cast support for the Userspace<T> pointer type

When using Userspace<T> there are certain syscalls where being able
to cast between types is needed. You should be able to easily cast
away the Userspace<T> wrapper, but it's perfectly safe to be able to
cast the internal type that is being wrapped.

AK/Userspace.h

@@ -73,6 +73,18 @@ private:
 #endif
 };
 
+template<typename T, typename U>
+inline Userspace<T> static_ptr_cast(const Userspace<U>& ptr)
+{
+#ifdef KERNEL
+    auto casted_ptr = static_cast<T>(ptr.unsafe_userspace_ptr());
+#else
+    auto casted_ptr = static_cast<T>(ptr.ptr());
+#endif
+    return Userspace<T>((FlatPtr)casted_ptr);
+}
+
 }
 
 using AK::Userspace;
+using AK::static_ptr_cast;