x86: fix memory setup bug

interesting...

[    0.000000]   mapped low ram: 0 - 20000000
[    0.000000]   low ram: 00000000 - 1fff0000
[    0.000000]   bootmap 00002000 - 00006000

max_pfn_mapped > max_low_pfn?

it seems init_memory_mapping reveals an old bug.

please check attached test patch.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/kernel/setup_32.c

@@ -427,7 +427,7 @@ void __init setup_arch(char **cmdline_p)
 	find_low_pfn_range();
 
 	/* max_pfn_mapped is updated here */
-	init_memory_mapping(0, (max_low_pfn << PAGE_SHIFT));
+	max_pfn_mapped = init_memory_mapping(0, (max_low_pfn << PAGE_SHIFT));
 
 	reserve_initrd();