fix iommu sg merging problem

IOMMUs merges scatter/gather segments without considering a low level
driver's restrictions. The problem is that IOMMUs can't access to the
limitations because they are in request_queue.

This patchset introduces a new structure, device_dma_parameters,
including dma information. A pointer to device_dma_parameters is added
to struct device. The bus specific structures (like pci_dev) includes
device_dma_parameters. Low level drivers can use dma_set_max_seg_size
to tell IOMMUs about the restrictions.

We can move more dma stuff in struct device (like dma_mask) to struct
device_dma_parameters later (needs some cleanups before that).

This includes patches for all the IOMMUs that could merge sg (x86_64,
ppc, IA64, alpha, sparc64, and parisc) though only the ppc patch was
tested. The patches for other IOMMUs are only compile tested.

Thanks to everyone for the comments on the previous submission
to linux-scsi.

This is against 2.6.24-rc1. The same patchset is also available:

git://git.kernel.org/pub/scm/linux/kernel/git/tomo/linux-2.6-misc.git iommu-sg-fixes
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