xen/swiotlb: relax alignment requirements

xen/swiotlb: relax alignment requirements

When mapping a buffer for DMA via .map_page or .map_sg DMA operations,
there is no need to check the machine frames to be aligned according
to the mapped areas size. All what is needed in these cases is that the
buffer is contiguous at machine level.

So carve out the alignment check from range_straddles_page_boundary()
and move it to a helper called by xen_swiotlb_alloc_coherent() and
xen_swiotlb_free_coherent() directly.

Fixes: 9f40ec84a797 ("xen/swiotlb: add alignment check for dma buffers")
Reported-by: Jan Vejvalka <jan.vejvalka@lfmotol.cuni.cz>
Tested-by: Jan Vejvalka <jan.vejvalka@lfmotol.cuni.cz>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Juergen Gross <jgross@suse.com>