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 v5.15.58 by commit 93fbc06da1d819f3981a7bd7928c3641ea67b364, it was introduced in version v3.14 by commit f87c10a8aa1e82498c42d0335524d6ae7cf5a52b. For more details please see the references link.