net/hsr: add interlink to fill_info output

Currently, it is possible to configure the interlink
port, but no way to read it back from userspace.

Add it to the output of hsr_fill_info(), so it can be
read from userspace, for example:

$ ip -d link show hsr0
12: hsr0: <BROADCAST,MULTICAST> mtu ...
...
hsr slave1 veth0 slave2 veth1 interlink veth2 ...

Signed-off-by: Jan Vaclav <[e-mail]>
Reviewed-by: Fernando Fernandez Mancera <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/hsr/hsr_netlink.c

···
 			goto nla_put_failure;
 	}
 
+	port = hsr_port_get_hsr(hsr, HSR_PT_INTERLINK);
+	if (port) {
+		if (nla_put_u32(skb, IFLA_HSR_INTERLINK, port->dev->ifindex))
+			goto nla_put_failure;
+	}
+
 	if (nla_put(skb, IFLA_HSR_SUPERVISION_ADDR, ETH_ALEN,
 		    hsr->sup_multicast_addr) ||
 	    nla_put_u16(skb, IFLA_HSR_SEQ_NR, hsr->sequence_nr))