iommu/vt-d: Fix scatterlist offset handling

The intel-iommu DMA ops fail to correctly handle scatterlists where
sg->offset is greater than PAGE_SIZE - the IOVA allocation is computed
appropriately based on the page-aligned portion of the offset, but the
mapping is set up relative to sg->page, which means it fails to actually
cover the whole buffer (and in the worst case doesn't cover it at all):

    (sg->dma_address + sg->dma_len) ----+
    sg->dma_address ---------+          |
    iov_pfn------+           |          |
                 |           |          |
                 v           v          v
iova:   a        b        c        d        e        f
        |--------|--------|--------|--------|--------|
                          <...calculated....>
                 [_____mapped______]
pfn:    0        1        2        3        4        5
        |--------|--------|--------|--------|--------|
                 ^           ^          ^
                 |           |          |
    sg->page ----+           |          |
    sg->offset --------------+          |
    (sg->offset + sg->length) ----------+

As a result, the caller ends up overrunning the mapping into whatever
lies beyond, which usually goes badly:

[  429.645492] DMAR: DRHD: handling fault status reg 2
[  429.650847] DMAR: [DMA Write] Request device [02:00.4] fault addr f2682000 ...

Whilst this is a fairly rare occurrence, it can happen from the result
of intermediate scatterlist processing such as scatterwalk_ffwd() in the
crypto layer. Whilst that particular site could be fixed up, it still
seems worthwhile to bring intel-iommu in line with other DMA API
implementations in handling this robustly.

To that end, fix the intel_map_sg() path to line up the mapping
correctly (in units of MM pages rather than VT-d pages to match the
aligned_nrpages() calculation) regardless of the offset, and use
sg_phys() consistently for clarity.

Reported-by: Harsh Jain <Harsh@chelsio.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed by: Ashok Raj <ashok.raj@intel.com>
Tested by: Jacob Pan <jacob.jun.pan@intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>