dma-direct: use the correct size for dma_set_encrypted()

The third parameter of dma_set_encrypted() is a size in bytes rather than
the number of pages.

Fixes: 4d056478

 ("dma-direct: factor out dma_set_{de,en}crypted helpers")
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>