ipv4: use RCU protection in __ip_rt_update_pmtu()

ipv4: use RCU protection in __ip_rt_update_pmtu()

[ Upstream commit 139512191bd06f1b496117c76372b2ce372c9a41 ]

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

Fixes: 2fbc6e89b2f1 ("ipv4: Update exception handling for multipath routes via same device")
Fixes: 1de6b15a434c ("Namespaceify min_pmtu sysctl")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250205155120.1676781-8-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>