src: add tunnel statement and expression support

src: add tunnel statement and expression support

This patch allows you to attach tunnel metadata through the tunnel
statement.

The following example shows how to redirect traffic to the erspan0
tunnel device which will take the tunnel configuration that is
specified by the ruleset.

     table netdev x {
            tunnel y {
                    id 10
                    ip saddr 192.168.2.10
                    ip daddr 192.168.2.11
                    sport 10
                    dport 20
                    ttl 10
                    erspan {
                            version 1
                            index 2
                    }
            }

	    chain x {
		    type filter hook ingress device veth0 priority 0;

		    ip daddr 10.141.10.123 tunnel name y fwd to erspan0
	    }
     }

This patch also allows to match on tunnel metadata via tunnel expression.

Joint work with Fernando.

Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>