mm: unexport globally copy_to_kernel_nofault

mm: unexport globally copy_to_kernel_nofault

copy_to_kernel_nofault() is an internal helper which should not be visible
to loadable modules – exporting it would give exploit code a cheap
oracle to probe kernel addresses.  Instead, keep the helper un-exported
and compile the kunit case that exercises it only when
mm/kasan/kasan_test.o is linked into vmlinux.

[snovitoll@gmail.com: add a brief comment to `#ifndef MODULE`]
Link: https://lkml.kernel.org/r/20250622141142.79332-1-snovitoll@gmail.com
Link: https://lkml.kernel.org/r/20250622051906.67374-1-snovitoll@gmail.com
Fixes: ca79a00bb9a8 ("kasan: migrate copy_user_test to kunit")
Signed-off-by: Sabyrzhan Tasbolatov <snovitoll@gmail.com>
Suggested-by: Christoph Hellwig <hch@infradead.org>
Suggested-by: Marco Elver <elver@google.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Dmitriy Vyukov <dvyukov@google.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>