netfilter: nft_osf: use NFT_OSF_MAXGENRELEN instead of IFNAMSIZ

As no "genre" on pf.os exceed 16 bytes of length, we reduce
NFT_OSF_MAXGENRELEN parameter to 16 bytes and use it instead of IFNAMSIZ.

Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>