[PATCH net] ip6_tunnel: Don't update PMTU on tunnels with collect_md

[Stefano Brivio <sbrivio@redhat.com>]

Commit 8d79266bc48c ("ip6_tunnel: add collect_md mode to IPv6
tunnels") introduced a check to avoid updating PMTU when
collect_md mode is enabled.

Later, commit f15ca723c1eb ("net: don't call update_pmtu
unconditionally") dropped this check, I guess inadvertently.
Restore it.

Fixes: f15ca723c1eb ("net: don't call update_pmtu unconditionally")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
---
 net/ipv6/ip6_tunnel.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/ip6_tunnel.c b/net/ipv6/ip6_tunnel.c
index a0b6932c3afd..6f05e0f74bdf 100644
--- a/net/ipv6/ip6_tunnel.c
+++ b/net/ipv6/ip6_tunnel.c
@@ -1136,7 +1136,8 @@ int ip6_tnl_xmit(struct sk_buff *skb, struct net_device *dev, __u8 dsfield,
 	mtu = max(mtu, skb->protocol == htons(ETH_P_IPV6) ?
 		       IPV6_MIN_MTU : IPV4_MIN_MTU);
 
-	skb_dst_update_pmtu(skb, mtu);
+	if (!t->parms.collect_md)
+		skb_dst_update_pmtu(skb, mtu);
 	if (skb->len - t->tun_hlen - eth_hlen > mtu && !skb_is_gso(skb)) {
 		*pmtu = mtu;
 		err = -EMSGSIZE;
-- 
2.19.1