rust: fix ARCH_SLAB_MINALIGN multiple definition error

We use const helpers in form of

    const size_t RUST_CONST_HELPER_ARCH_SLAB_MINALIGN = ARCH_SLAB_MINALIGN;

to aid generation of constants by bindgen because it is otherwise a
macro definition of an expression and bindgen doesn't expand the
constant. The helpers are then have `RUST_CONST_HELPER` prefix stripped
and exposed to Rust code as if `ARCH_SLAB_MISALIGN` is generated
natively by bindgen.

This works well for most constants, but on RISC-V, `ARCH_SLAB_MINALIGN`
is defined directly as literal constant if `!CONFIG_MMU`, and bindgen
would generate `ARCH_SLAB_MINALIGN` directly, thus conflict with the
one generated through the helper.

To fix this, we simply need to block bindgen from generating directly
without going through helper.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202409160804.eSg9zh1e-lkp@intel.com/
Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Christian Schrefl <chrisi.schrefl@gmail.com>
Link: https://lore.kernel.org/r/20240916003347.1744345-1-gary@garyguo.net
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/bindgen_parameters

@@ -24,3 +24,8 @@
 # These functions use the `__preserve_most` calling convention, which neither bindgen
 # nor Rust currently understand, and which Clang currently declares to be unstable.
 --blocklist-function __list_.*_report
+
+# These constants are sometimes not recognized by bindgen depending on config.
+# We use const helpers to aid bindgen, to avoid conflicts when constants are
+# recognized, block generation of the non-helper constants.
+--blocklist-item ARCH_SLAB_MINALIGN