ipv4: use RCU protection in ipv4_default_advmss()

[ Upstream commit 71b8471c93fa0bcab911fcb65da1eb6c4f5f735f ]

ipv4_default_advmss() must use RCU protection to make
sure the net structure it reads does not disappear.

Fixes: 2e9589ff ("ipv4: Namespaceify min_adv_mss sysctl knob")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250205155120.1676781-5-edumazet@google.com


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