tcp: Fix a data-race around sysctltcpmtuprobefloor.
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 d5bece4df6090395f891110ef52a6f82d16685db, it was introduced in version v5.4 by commit c04b79b6cfd714144f6a2cf359603d82ee631e62. For more details please see the references link.