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 v5.10.134 by commit b96ed5ccb09ae71103023ed13acefb194f609794, it was introduced in version v3.14 by commit f87c10a8aa1e82498c42d0335524d6ae7cf5a52b. For more details please see the references link.