tunnel: add missing tunnel object list support

tunnel: add missing tunnel object list support

Tunnel object listing support was missing. Now it is possible to list
tunnels. Example:

sudo nft list tunnel netdev x y
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
		}
	}
}

Fixes: a937a5dc02db ("src: add tunnel statement and expression support")
Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Signed-off-by: Florian Westphal <fw@strlen.de>