powerpc/book3s64/mm: Use PAGE_KERNEL instead of opencoding

No functional change in this patch.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Tested-by: Sachin Sant &lt;sachinp@linux.ibm.com <mailto:sachinp@linux.ibm.com&gt;>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230616110826.344417-5-aneesh.kumar@linux.ibm.com