loadpin: simplify sysctls use with register_sysctl()

register_sysctl_paths() is not required, we can just use
register_sysctl() with the required path specified.

Reviewed-by: John Johansen <john.johansen@canonical.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>