x86: extend e820 early_res support 32bit -fix #3

introduce init_pg_table_start, so xen PV could specify the value.

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/arch/x86/kernel/head32.c b/arch/x86/kernel/head32.c
index c216d3c..0b6cb9f 100644
--- a/arch/x86/kernel/head32.c
+++ b/arch/x86/kernel/head32.c
@@ -76,7 +76,8 @@
 		reserve_early(ramdisk_image, ramdisk_end, "RAMDISK");
 	}
 #endif
-	reserve_early(__pa_symbol(&_end), init_pg_tables_end, "INIT_PG_TABLE");
+	reserve_early(init_pg_tables_start, init_pg_tables_end,
+			"INIT_PG_TABLE");
 
 	reserve_ebda_region();