x86: max_low_pfn_mapped fix, #1

fix crash on Ingo's big box:

calling  pci_iommu_init+0x0/0x17
PCI-DMA: Disabling AGP.
PCI-DMA: aperture base @ d0000000 size 65536 KB
PCI-DMA: using GART IOMMU.
PCI-DMA: Reserving 64MB of IOMMU area in the AGP aperture
BUG: unable to handle kernel paging request at ffff88000003be88
IP: [<ffffffff8026d377>] __alloc_pages_internal+0xc3/0x3f2
PGD 202063 PUD 206063 PMD 22fc00163 PTE 3b162
Oops: 0000 [1] SMP

and e820 is:

 BIOS-e820: 0000000000000000 - 000000000009ac00 (usable)
 BIOS-e820: 000000000009ac00 - 00000000000a0000 (reserved)
 BIOS-e820: 00000000000ca000 - 0000000000100000 (reserved)
 BIOS-e820: 0000000000100000 - 000000007ff70000 (usable)
 BIOS-e820: 000000007ff70000 - 000000007ff86000 (ACPI data)
 BIOS-e820: 000000007ff86000 - 0000000080000000 (ACPI NVS)
 BIOS-e820: 0000000080000000 - 00000000cfe00000 (usable)
 BIOS-e820: 00000000cfe00000 - 00000000d0000000 (reserved)
 BIOS-e820: 00000000e0000000 - 00000000f0000000 (reserved)
 BIOS-e820: 00000000fec00000 - 00000000fec10000 (reserved)
 BIOS-e820: 00000000fee00000 - 00000000fee01000 (reserved)
 BIOS-e820: 00000000fff80000 - 0000000100000000 (reserved)
 BIOS-e820: 0000000100000000 - 0000000830000000 (usable)

system has 32 GB RAM installed.

max_low_pfn_mapped is 0xcfe00, and GART aperture is not mapped.

So try to use init_memory_mapping to map that area, because the iommu
thinks that area is ram ...

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>