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.

[[e-mail]: drop rust part of comment for _copy_from_user(), per Alice]
Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Yury Norov (NVIDIA) <[e-mail]>
Acked-by: Arnd Bergmann <[e-mail]>
Acked-by: Miguel Ojeda <[e-mail]>
Reviewed-by: Alice Ryhl <[e-mail]>
Tested-by: Alice Ryhl <[e-mail]>
Cc: Alex Gaynor <[e-mail]>
Cc: Andreas Hindborg <[e-mail]>
Cc: Björn Roy Baron <[e-mail]>
Cc: Boqun Feng <[e-mail]>
Cc: Danilo Krummrich <[e-mail]>
Cc: Gary Guo <[e-mail]>
Cc: John Hubbard <[e-mail]>
Cc: Trevor Gross <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/uaccess.h

···
  * directly in the normal copy_to/from_user(), the other ones go
  * through an extern _copy_to/from_user(), which expands the same code
  * here.
- *
- * Rust code always uses the extern definition.
  */
 static inline __must_check unsigned long
 _inline_copy_from_user(void *to, const void __user *from, unsigned long n)

lib/usercopy.c

···
 
 /* out-of-line parts */
 
-#if !defined(INLINE_COPY_FROM_USER) || defined(CONFIG_RUST)
+#if !defined(INLINE_COPY_FROM_USER)
 unsigned long _copy_from_user(void *to, const void __user *from, unsigned long n)
 {
 	return _inline_copy_from_user(to, from, n);
···
 EXPORT_SYMBOL(_copy_from_user);
 #endif
 
-#if !defined(INLINE_COPY_TO_USER) || defined(CONFIG_RUST)
+#if !defined(INLINE_COPY_TO_USER)
 unsigned long _copy_to_user(void __user *to, const void *from, unsigned long n)
 {
 	return _inline_copy_to_user(to, from, n);

rust/helpers/uaccess.c

···
 {
 	return copy_to_user(to, from, n);
 }
+
+#ifdef INLINE_COPY_FROM_USER
+unsigned long rust_helper__copy_from_user(void *to, const void __user *from, unsigned long n)
+{
+	return _inline_copy_from_user(to, from, n);
+}
+
+unsigned long rust_helper__copy_to_user(void __user *to, const void *from, unsigned long n)
+{
+	return _inline_copy_to_user(to, from, n);
+}
+#endif