x86, 64-bit: use __pgd() on mk_kernel_pgd()

Use __pgd() on mk_kernel_pgd()

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Cc: xen-devel <xen-devel@lists.xensource.com>
Cc: Stephen Tweedie <sct@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>