swiotlb: respect min_align_mask

Respect the min_align_mask in struct device_dma_parameters in swiotlb.

There are two parts to it:
 1) for the lower bits of the alignment inside the io tlb slot, just
    extent the size of the allocation and leave the start of the slot
     empty
 2) for the high bits ensure we find a slot that matches the high bits
    of the alignment to avoid wasting too much memory

Based on an earlier patch from Jianxiong Gao <jxgao@google.com>.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Jianxiong Gao <jxgao@google.com>
Tested-by: Jianxiong Gao <jxgao@google.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>