dma-iommu: Check that swiotlb is active before trying to use it

If the IOMMU is in use and an untrusted device is connected to an external
facing port but the address requested isn't page aligned will cause the
kernel to attempt to use bounce buffers.

If for some reason the bounce buffers have not been allocated this is a
problem that should be made apparent to the user.

Signed-off-by: Mario Limonciello <[email protected]>
Reviewed-by: Robin Murphy <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Joerg Roedel <[email protected]>

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