x86, 64bit: #PF handler set page to cover only 2M per #PF

We only map a single 2 MiB page per #PF, even though we should be able
to do this a full gigabyte at a time with no additional memory cost.
This is a workaround for a broken AMD reference BIOS (and its
derivatives in shipping system) which maps a large chunk of memory as
WB in the MTRR system but will #MC if the processor wanders off and
tries to prefetch that memory, which can happen any time the memory is
mapped in the TLB.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/1359058816-7615-13-git-send-email-yinghai@kernel.org
Cc: Alexander Duyck <alexander.h.duyck@intel.com>
[ hpa: rewrote the patch description ]
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>