uaccess: decouple INLINE_COPY_FROM_USER and CONFIG_RUST

uaccess: decouple INLINE_COPY_FROM_USER and CONFIG_RUST

Commit 1f9a8286bc0c ("uaccess: always export _copy_[from|to]_user with
CONFIG_RUST") exports _copy_{from,to}_user() unconditionally, if RUST is
enabled.  This pollutes exported symbols namespace, and spreads RUST
ifdefery in core files.

It's better to declare a corresponding helper under the rust/helpers,
similarly to how non-underscored copy_{from,to}_user() is handled.

[yury.norov@gmail.com: drop rust part of comment for _copy_from_user(), per Alice]
Link: https://lkml.kernel.org/r/20251024154754.99768-1-yury.norov@gmail.com
Link: https://lkml.kernel.org/r/20251023171607.1171534-1-yury.norov@gmail.com
Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Alice Ryhl <aliceryhl@google.com>
Cc: Alex Gaynor <alex.gaynor@gmail.com>
Cc: Andreas Hindborg <a.hindborg@kernel.org>
Cc: Björn Roy Baron <bjorn3_gh@protonmail.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: Gary Guo <gary@garyguo.net>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Trevor Gross <tmgross@umich.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>