ipv4: use RCU protection in ip_dst_mtu_maybe_forward()

ipv4: use RCU protection in ip_dst_mtu_maybe_forward()

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

Fixes: f87c10a8aa1e8 ("ipv4: introduce ip_dst_mtu_maybe_forward and protect forwarding path against pmtu spoofing")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250205155120.1676781-4-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>