Linux 5.12's very bad, double ungood day

Crazy idea: require kernel.testing.alpha=5.20.0-alpha1 on the cmdline to boot such an alpha kernel. Reject such an option in non-alpha kernels (including development kernels; the tagged release would require this code and not otherwise).

But this kind of one-off code is annoying to test itself and someone will script adding it to their boot command lines anyways.