l2tp: fix manual sequencing (de)activation in L2TPv2

Commit e0d4435f "l2tp: Update PPP-over-L2TP driver to work over L2TPv3"
broke the PPPOL2TP_SO_SENDSEQ setsockopt. The L2TP header length was
previously computed by pppol2tp_l2t_header_len() before each call to
l2tp_xmit_skb(). Now that header length is retrieved from the hdr_len
session field, this field must be updated every time the L2TP header
format is modified, or l2tp_xmit_skb() won't push the right amount of
data for the L2TP header.

This patch uses l2tp_session_set_header_len() to adjust hdr_len every
time sequencing is (de)activated from userspace (either by the
PPPOL2TP_SO_SENDSEQ setsockopt or the L2TP_ATTR_SEND_SEQ netlink
attribute).

Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/l2tp/l2tp_netlink.c b/net/l2tp/l2tp_netlink.c
index 4cfd722..bd7387a 100644
--- a/net/l2tp/l2tp_netlink.c
+++ b/net/l2tp/l2tp_netlink.c
@@ -578,8 +578,10 @@
 	if (info->attrs[L2TP_ATTR_RECV_SEQ])
 		session->recv_seq = nla_get_u8(info->attrs[L2TP_ATTR_RECV_SEQ]);
 
-	if (info->attrs[L2TP_ATTR_SEND_SEQ])
+	if (info->attrs[L2TP_ATTR_SEND_SEQ]) {
 		session->send_seq = nla_get_u8(info->attrs[L2TP_ATTR_SEND_SEQ]);
+		l2tp_session_set_header_len(session, session->tunnel->version);
+	}
 
 	if (info->attrs[L2TP_ATTR_LNS_MODE])
 		session->lns_mode = nla_get_u8(info->attrs[L2TP_ATTR_LNS_MODE]);