netfilter: support for element deletion

netfilter: support for element deletion

This patch implements element deletion from ruleset.

Example:

	table ip set-test {
		set testset {
			type ipv4_addr;
			flags timeout;
		}

		chain outputchain {
			policy accept;
			type filter hook output priority filter;

			delete @testset { ip saddr }
		}
	}

Signed-off-by: Ander Juaristi <a@juaristi.eus>
Signed-off-by: Florian Westphal <fw@strlen.de>