src: support for restoring element counters

src: support for restoring element counters

This patch allows you to restore counters in dynamic sets:

 table ip test {
        set test {
                type ipv4_addr
                size 65535
                flags dynamic,timeout
                timeout 30d
                gc-interval 1d
                elements = { 192.168.10.13 expires 19d23h52m27s576ms counter packets 51 bytes 17265 }
        }
        chain output {
                type filter hook output priority 0;
                update @test { ip saddr }
        }
 }

You can also add counters to elements from the control place, ie.

 table ip test {
        set test {
                type ipv4_addr
                size 65535
                elements = { 192.168.2.1 counter packets 75 bytes 19043 }
        }

        chain output {
                type filter hook output priority filter; policy accept;
                ip daddr @test
        }
 }

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