powerpc/copro: Fix faulting kernel segments

This fixes calculating the key bits (KP and KS) in the SLB VSID for kernel
mappings.

I'm not CCing this to stable as there are no uses of this currently.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>