netfilter: ipset: refactor deprecated strncpy

Use `strscpy_pad` instead of `strncpy`.

Link: https://github.com/KSPP/linux/issues/90


Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>