netfilter: nft_limit: fix stateful object memory leak

We need to provide a destroy callback to release the extra fields.

Fixes: 3b9e2ea6

 ("netfilter: nft_limit: move stateful fields out of expression data")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>