netfilter: nft_fib: convert htonl to ntohl properly

Acctually ntohl and htonl are identical, so this doesn't affect
anything, but it is conceptually wrong.

Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>