[PATCH 6.1.y 3/4] ipv6: use dst6_mtu() instead of dst_mtu()

[Sasha Levin <sashal@kernel.org>]

From: Eric Dumazet <edumazet@google.com>

[ Upstream commit b40f0130a23b6bab8b359e158a40af81f11f14be ]

When we expect an IPv6 dst, use dst6_mtu() instead of dst_mtu()
to save some code space.

Due to current dst6_mtu() implementation, only convert
users in IPv6 stack.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260130210303.3888261-7-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

[ sashal: Reduced backport -- upstream b40f0130a23b6 touches 6 file(s), this
  backport carries 1. Not backported here:
  net/ipv6/ip6_gre.c
  net/ipv6/ip6_output.c
  net/ipv6/ip6_tunnel.c
  net/ipv6/ipv6_sockglue.c
  net/ipv6/route.c
  net/ipv6/tcp_ipv6.c
  This note is generated from the file lists only; see the resolution record
  for the reasoning. ]

Stable-dep-of: 2640e6419594 ("net: advertise TCP MSS from the configured MTU, not the learned PMTU")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 include/net/ip.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/net/ip.h b/include/net/ip.h
index 1b0722e9b55e4..f44808d6f420b 100644
--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -453,7 +453,7 @@ static inline unsigned int ip_dst_mtu_maybe_forward(const struct dst_entry *dst,
 {
 	const struct rtable *rt = container_of(dst, struct rtable, dst);
 	struct net *net = dev_net(dst->dev);
-	unsigned int mtu;
+	unsigned int mtu, res;
 
 	if (READ_ONCE(net->ipv4.sysctl_ip_fwd_use_pmtu) ||
 	    ip_mtu_locked(dst) ||
@@ -477,8 +477,9 @@ static inline unsigned int ip_dst_mtu_maybe_forward(const struct dst_entry *dst,
 
 out:
 	mtu = min_t(unsigned int, mtu, IP_MAX_MTU);
+	res = mtu - lwtunnel_headroom(dst->lwtstate, mtu);
 
-	return mtu - lwtunnel_headroom(dst->lwtstate, mtu);
+	return res;
 }
 
 static inline unsigned int ip_skb_dst_mtu(struct sock *sk,
-- 
2.53.0