x86_64 don't expose the intermediate representation with one underline,
_PAGE_KERNEL, just the double-underlined one.

Use it, to get a common ground between 32 and 64-bit
Signed-off-by: NGlauber de Oliveira Costa <gcosta@redhat.com>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>