kallsyms: use `ARRAY_SIZE` instead of hardcoded size

This removes one place where the `500` constant is hardcoded.

Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Geert Stappers <stappers@stappers.nl>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Co-developed-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>