x86/kexec: Allocate PGD for x86_64 transition page tables separately

x86/kexec: Allocate PGD for x86_64 transition page tables separately

Now that commit d0ceea662d45 ("x86/mm: Add _PAGE_NOPTISHADOW bit to
avoid updating userspace page tables") stops kernel_ident_mapping_init()
from scribbling over the end of a 4KiB PGD by assuming the following
4KiB will be a userspace PGD, there's no good reason for the kexec PGD
to be part of a single 8KiB allocation with the control_code_page.

(It's not clear that that was the reason for x86_64 kexec doing it that
way in the first place either; there were no comments to that effect and
it seems to have been the case even before PTI came along. It looks like
it was just a happy accident which prevented memory corruption on kexec.)

Either way, it definitely isn't needed now. Just allocate the PGD
separately on x86_64, like i386 already does.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>