tunnel: add erspan support

tunnel: add erspan support

This patch extends the tunnel metadata object to define erspan tunnel
specific configurations:

 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
                }
        }
 }

Joint work with Fernando.

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