Backports that remove FPU lazy-mode deadcode for 4.9.y

Hello Greg,

Please consider this backport for 4.9.y. It completes the removal of FPU lazy-mode code, documentation and comments.

[PATCH v3 4.9] x86/fpu: Remove use_eager_fpu()

After applying the patch, please do the following: - cherry-pick: 3913cc350757 ("x86/fpu: Remove struct fpu::counter") - revert: f09a7b0eead7 ("perf: sync up x86/.../cpufeatures.h") - Because the modification in this patch actually belongs to e63650840e8b ("x86/fpu: Finish excising 'eagerfpu'") - cherry-pick: e63650840e8b ("x86/fpu: Finish excising 'eagerfpu'")

Thanks, Daniel