xen: defer building p2m mfn structures until kernel is mapped

When building mfn parts of p2m structure, we rely on being able to
use mfn_to_virt, which in turn requires kernel to be mapped into
the linear area (which is distinct from the kernel image mapping
on 64-bit).  Defer calling xen_build_mfn_list_list() until after
xen_setup_kernel_pagetable();
Signed-off-by: NJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

arch/x86/xen/enlighten.c

@@ -1178,6 +1178,9 @@ asmlinkage void __init xen_start_kernel(void)
 	xen_raw_console_write("mapping kernel into physical memory\n");
 	pgd = xen_setup_kernel_pagetable(pgd, xen_start_info->nr_pages);
 
+	/* Allocate and initialize top and mid mfn levels for p2m structure */
+	xen_build_mfn_list_list();
+
 	init_mm.pgd = pgd;
 
 	/* keep using Xen gdt for now; no urgent need to change it */

arch/x86/xen/mmu.c

@@ -374,9 +374,6 @@ void __init xen_build_dynamic_phys_to_machine(void)
 
 		p2m_top[topidx][mididx] = &mfn_list[pfn];
 	}
-
-	/* Allocate and initialize top and mid mfn levels */
-	xen_build_mfn_list_list();
 }
 
 unsigned long get_phys_to_machine(unsigned long pfn)