ipv4: use RCU protection in rt_is_expired()

ipv4: use RCU protection in rt_is_expired()

[ Upstream commit dd205fcc33d92d54eee4d7f21bb073af9bd5ce2b ]

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

Fixes: e84f84f27647 ("netns: place rt_genid into struct net")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250205155120.1676781-6-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>