lguest: use __PAGE_KERNEL instead of _PAGE_KERNEL

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: Glauber de Oliveira Costa <gcosta@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>