ipv6: use RCU protection in ip6_default_advmss()

[ Upstream commit 3c8ffcd248da34fc41e52a46e51505900115fc2a ]

ip6_default_advmss() needs rcu protection to make
sure the net structure it reads does not disappear.

Fixes: 5578689a ("[NETNS][IPV6] route6 - make route6 per namespace")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250205155120.1676781-11-edumazet@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>