ip: Fix data-races around sysctlipfwdusepmtu.
This is an automated ID intended to aid in discovery of potential security vulnerabilities. The actual impact and attack plausibility have not yet been proven. This ID is fixed in Linux Kernel version v5.4.208 by commit 7828309df0f89419a9349761a37c7d1b0da45697, it was introduced in version v3.14 by commit f87c10a8aa1e82498c42d0335524d6ae7cf5a52b. For more details please see the references link.