netfilter: nft_numgen: rename until attribute by modulus

The _until_ attribute is renamed to _modulus_ as the behaviour is similar to
other expresions with number limits (ex. nft_hash).

Renaming is possible because there isn't a kernel release yet with these
changes.

Signed-off-by: Laura Garcia Liebana <nevola@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>