netlink: improve rule deletion per chain

netlink: improve rule deletion per chain

With this patch, nft asks the kernel for deleting all rules in a chain.
This replaces the current behaviour that requires to dump all the rules,
then iterate over that list to delete one by one, which is prone to races
and slowier.

After this patch, the following two commands are equivalent:

 nft flush chain filter input
 nft delete rule filter input

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>