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: Ingo Molnar <mingo@elte.hu>