scripts: checkpatch: enable language-independent checks for Rust

Include Rust in the "source code files" category, so that
the language-independent tests are checked for Rust too,
and teach `checkpatch` about the comment style for Rust files.

This enables the malformed SPDX check, the misplaced SPDX license
tag check, the long line checks, the lines without a newline check
and the embedded filename check.

Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Co-developed-by: Alex Gaynor <alex.gaynor@gmail.com>
Signed-off-by: Alex Gaynor <alex.gaynor@gmail.com>
Co-developed-by: Wedson Almeida Filho <wedsonaf@google.com>
Signed-off-by: Wedson Almeida Filho <wedsonaf@google.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>