real(0.1,cvmix_r8) is not the same as 0.1_cvmix_r8

I thought these two calls were equivalent, but even though the output of both
calls is kind=cvmix_r8, the former has round-off error at the default kind.
I.e. 0.1_cvmix_r8 - real(0.1, cvmix_r8) = O(1e-9)