netfilter: nft_osf: refactor deprecated strncpy

Use `strscpy_pad` over `strncpy` for NUL-terminated strings.

We can also drop the + 1 from `NFT_OSF_MAXGENRELEN + 1` since `strscpy`
will guarantee NUL-termination.

Signed-off-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>