xen: fix dom0 boot on huge systems

xen: fix dom0 boot on huge systems

Commit f7c90c2aa40048 ("x86/xen: don't write ptes directly in 32-bit
PV guests") introduced a regression for booting dom0 on huge systems
with lots of RAM (in the TB range).

Reason is that on those hosts the p2m list needs to be moved early in
the boot process and this requires temporary page tables to be created.
Said commit modified xen_set_pte_init() to use a hypercall for writing
a PTE, but this requires the page table being in the direct mapped
area, which is not the case for the temporary page tables used in
xen_relocate_p2m().

As the page tables are completely written before being linked to the
actual address space instead of set_pte() a plain write to memory can
be used in xen_relocate_p2m().

Fixes: f7c90c2aa40048 ("x86/xen: don't write ptes directly in 32-bit PV guests")
Cc: stable@vger.kernel.org
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>