netfilter: nft_limit: Clone packet limits' cost value

When cloning a packet-based limit expression, copy the cost value as
well. Otherwise the new limit is not functional anymore.

Fixes: 3b9e2ea6 ("netfilter: nft_limit: move stateful fields out of expression data")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>