KVM: PPC: Book3S HV: Use RADIX_PTE_INDEX_SIZE in Radix MMU code

In kvmppc_unmap_free_pte() in book3s_64_mmu_radix.c, we use the
non-constant value PTE_INDEX_SIZE to clear a PTE page.

We can instead use the constant RADIX_PTE_INDEX_SIZE, because we know
this code will only be running when the Radix MMU is active.

Note that we already use RADIX_PTE_INDEX_SIZE for the allocation of
kvm_pte_cache.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Leonardo Bras <leonardo@linux.ibm.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>