nft: Extend use of nftnl_chain_list_foreach()

nft: Extend use of nftnl_chain_list_foreach()

Make use of the callback-based iterator in nft_rule_list(),
nft_rule_list_save(), nft_rule_flush() and nft_rule_save().

Callback code for nft_rule_list() and nft_rule_list_save is pretty
similar, so introduce and use a common callback function.

For nft_rule_save(), turn nft_chain_save_rules() into a callback - it is
not used anywhere else, anyway.

Signed-off-by: Phil Sutter <phil@nwl.cc>