l2tp: fix missing print session offset info

Report offset parameter in L2TP_CMD_SESSION_GET command if
it has been configured by userspace

Fixes: 309795f4 ("l2tp: Add netlink control API for L2TP")
Reported-by: NJianlin Shi <jishi@redhat.com>
Signed-off-by: NHangbin Liu <liuhangbin@gmail.com>
Signed-off-by: NLorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/l2tp/l2tp_netlink.c

@@ -761,6 +761,8 @@ static int l2tp_nl_session_send(struct sk_buff *skb, u32 portid, u32 seq, int fl
 
 	if ((session->ifname[0] &&
 	     nla_put_string(skb, L2TP_ATTR_IFNAME, session->ifname)) ||
+	    (session->offset &&
+	     nla_put_u16(skb, L2TP_ATTR_OFFSET, session->offset)) ||
 	    (session->cookie_len &&
 	     nla_put(skb, L2TP_ATTR_COOKIE, session->cookie_len,
 		     &session->cookie[0])) ||