In my opinion, this is not a valid way to license software under the GPL.
This isn't a valid way to license software period. The Linux kernel is in legal mud waters. There are many files in it that violate the GPL, but since nobody is capable of enforcing the copyright of Linux, nobody can really do anything.
What rights do I have to use the source code in the kernel tree which has not specified a license?
If you take it out from Linux, then GPLv2, since that is how the whole kernel licensed. If you would find the file alone somewhere, then it depends on the notice in the license. If no notice, no right to even look at it.
Nobody can change the license of Linux, there are thousands of copyright holders, and to be able to do such a license change, you need to have _all_ copyright holder to agree to the change. This is impratical.
Cheers.