FROMLIST: uaccess: always export _copy_[from|to]_user with CONFIG_RUST

Rust code needs to be able to access _copy_from_user and _copy_to_user
so that it can skip the check_copy_size check in cases where the length
is known at compile-time, mirroring the logic for when C code will skip
check_copy_size. To do this, we ensure that exported versions of these
methods are available when CONFIG_RUST is enabled.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Bug: 324206405
Link: https://lore.kernel.org/all/20240208-alice-mm-v2-2-d821250204a6@google.com/


Change-Id: If675094b382a9a6ae446888d1d662b46ae746ae6
[ Add include of nospec.h for definition of barrier_nospec. ]
Signed-off-by: Alice Ryhl <aliceryhl@google.com>