xen/setup: Fix for incorrect xen_extra_mem_start initialization under 32-bit

git commit 24bdb0b6 (xen: do not create
the extra e820 region at an addr lower than 4G) does not take into
account that ifdef CONFIG_X86_32 instead of e820_end_of_low_ram_pfn()
find_low_pfn_range() is called (both calls are from arch/x86/kernel/setup.c).
find_low_pfn_range() behaves correctly and does not require change in
xen_extra_mem_start initialization. Additionally, if xen_extra_mem_start
is initialized in the same way as ifdef CONFIG_X86_64 then memory hotplug
support for Xen balloon driver (under development) is broken.
Signed-off-by: Daniel Kiper <dkiper@net-space.pl>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/xen/setup.c

@@ -227,7 +227,11 @@ char * __init xen_memory_setup(void)
 
 	memcpy(map_raw, map, sizeof(map));
 	e820.nr_map = 0;
+#ifdef CONFIG_X86_32
+	xen_extra_mem_start = mem_end;
+#else
 	xen_extra_mem_start = max((1ULL << 32), mem_end);
+#endif
 	for (i = 0; i < memmap.nr_entries; i++) {
 		unsigned long long end;