xen: allow compilation with non-flat memory

There's no real reason we can't support sparsemem/discontigmem, so do so.
This is mostly useful to support hotplug memory.
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/xen/Kconfig

@@ -6,7 +6,7 @@ config XEN
 	bool "Xen guest support"
 	select PARAVIRT
 	depends on X86_32
-	depends on X86_CMPXCHG && X86_TSC && !NEED_MULTIPLE_NODES && !(X86_VISWS || X86_VOYAGER)
+	depends on X86_CMPXCHG && X86_TSC && !(X86_VISWS || X86_VOYAGER)
 	help
 	  This is the Linux Xen port.  Enabling this will allow the
 	  kernel to boot in a paravirtualized environment under the

arch/x86/xen/enlighten.c

@@ -669,7 +669,9 @@ static void xen_write_cr3(unsigned long cr3)
    everything is pinned. */
 static __init void xen_alloc_pte_init(struct mm_struct *mm, u32 pfn)
 {
+#ifdef CONFIG_FLATMEM
 	BUG_ON(mem_map);	/* should only be used early */
+#endif
 	make_lowmem_page_readonly(__va(PFN_PHYS(pfn)));
 }