powerpc/32s: map kasan zero shadow with PAGE_READONLY instead of PAGE_KERNEL_RO

For hash32, the zero shadow page gets mapped with PAGE_READONLY instead
of PAGE_KERNEL_RO, because the PP bits don't provide a RO kernel, so
PAGE_KERNEL_RO is equivalent to PAGE_KERNEL. By using PAGE_READONLY,
the page is RO for both kernel and user, but this is not a security issue
as it contains only zeroes.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>