ix86: fix linear page table construction in alloc_l2_table()

Slot 0 got updated when slot 3 was meant. The mistake was hidden by
create_pae_xen_mappings() correcting things immediately afterwards
(i.e. before the new entries could get used the first time).

Reported-by: CHENG Yueqiang <yqcheng.2008@xxxxxxxxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -1488,7 +1488,7 @@ static int alloc_l2_table(struct page_in
             l2e_write(&pl2e[l2_table_offset(PERDOMAIN_VIRT_START) + i],
                       l2e_from_page(perdomain_pt_page(d, i),
                                     __PAGE_HYPERVISOR));
-        pl2e[l2_table_offset(LINEAR_PT_VIRT_START)] =
+        pl2e[l2_table_offset(LINEAR_PT_VIRT_START) + 3] =
             l2e_from_pfn(pfn, __PAGE_HYPERVISOR);
 #else
         memcpy(&pl2e[COMPAT_L2_PAGETABLE_FIRST_XEN_SLOT(d)],