rust: io: always inline functions using build_assert with arguments

rust: io: always inline functions using build_assert with arguments

`build_assert` relies on the compiler to optimize out its error path.
Functions using it with its arguments must thus always be inlined,
otherwise the error path of `build_assert` might not be optimized out,
triggering a build error.

Cc: stable@vger.kernel.org
Fixes: ce30d94e6855 ("rust: add `io::{Io, IoRaw}` base types")
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Tested-by: Timur Tabi <ttabi@nvidia.com>
Link: https://patch.msgid.link/20251208-io-build-assert-v3-2-98aded02c1ea@nvidia.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>