xen/mmu: Move the setting of pvops.write_cr3 to later phase in bootup.

We move the setting of write_cr3 from the early bootup variant
(see git commit 0cc9129d
"x86-64, xen, mmu: Provide an early version of write_cr3.")
to a more appropiate location.

This new location sets all of the other non-early variants
of pvops calls - and most importantly is before the
alternative_asm mechanism kicks in.
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/xen/mmu.c

@@ -1467,8 +1467,6 @@ static void __init xen_write_cr3_init(unsigned long cr3)
 	__xen_write_cr3(true, cr3);
 
 	xen_mc_issue(PARAVIRT_LAZY_CPU);  /* interrupts restored */
-
-	pv_mmu_ops.write_cr3 = &xen_write_cr3;
 }
 #endif
 
@@ -2122,6 +2120,7 @@ static void __init xen_post_allocator_init(void)
 #endif
 
 #ifdef CONFIG_X86_64
+	pv_mmu_ops.write_cr3 = &xen_write_cr3;
 	SetPagePinned(virt_to_page(level3_user_vsyscall));
 #endif
 	xen_mark_init_mm_pinned();