kaiser: allocate pgd with order 0 when pti=off

kaiser: allocate pgd with order 0 when pti=off

The 4.9.77 version of "x86/pti/efi: broken conversion from efi to kernel
page table" looked nicer than the 4.4.112 version, but was suboptimal on
machines booted with "pti=off" (or on AMD machines): it allocated pgd
with an order 1 page whatever the setting of kaiser_enabled.

Fix that by moving the definition of PGD_ALLOCATION_ORDER from
asm/pgalloc.h to asm/pgtable.h, which already defines kaiser_enabled.

Fixes: 1b92c48a2eeb ("x86/pti/efi: broken conversion from efi to kernel page table")
Reviewed-by: Pavel Tatashin <pasha.tatashin@oracle.com>
Cc: Steven Sistare <steven.sistare@oracle.com>
Cc: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>