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.

Kernel/Syscalls/read.cpp

@@ -57,7 +57,7 @@ ssize_t Process::sys$read(int fd, Userspace<u8*> buffer, ssize_t size)
         }
     }
     // FIXME: We should have a read() that takes a Userspace<u8*>
-    return description->read((u8*)buffer.ptr(), size);
+    return description->read(buffer.unsafe_userspace_ptr(), size);
 }
 
 }