In this particular case it is not viral. All MIT-licensed code can be "licensed as a whole at no charge to all third parties under" GPL 2. The point of the MIT license is that it can be freely relicensed.

You actually have more rights with that file than with the others; the GPL doesn't restrict non-GPL'd code distributed with it, it requires that that code be licensed in a way that allows it to be distributed under GPL 2. For example, I myself can take this code and redistribute the entire thing under GPL 2 without an exception for ./kernel/mem.c (as long as I include any acknowledgement required by MIT, such as a copy of the MIT license and a copyright notice.)

You're overcomplicating things and (maybe accidentally) spreading FUD. MIT was specifically designed to be compatible with anything from copyleft to proprietary.