swiotlb: fix phys_addr_t overflow warning

On architectures that have a larger dma_addr_t than phys_addr_t,
the swiotlb_tbl_map_single() function truncates its return code
in the failure path, making it impossible to identify the error
later, as we compare to the original value:

kernel/dma/swiotlb.c:551:9: error: implicit conversion from 'dma_addr_t' (aka 'unsigned long long') to 'phys_addr_t' (aka 'unsigned int') changes value from 18446744073709551615 to 4294967295 [-Werror,-Wconstant-conversion]
        return DMA_MAPPING_ERROR;

Use an explicit typecast here to convert it to the narrower type,
and use the same expression in the error handling later.

Fixes: b907e20508 ("swiotlb: remove SWIOTLB_MAP_ERROR")
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/swiotlb-xen.c

@@ -408,7 +408,7 @@ static dma_addr_t xen_swiotlb_map_page(struct device *dev, struct page *page,
 
 	map = swiotlb_tbl_map_single(dev, start_dma_addr, phys, size, dir,
 				     attrs);
-	if (map == DMA_MAPPING_ERROR)
+	if (map == (phys_addr_t)DMA_MAPPING_ERROR)
 		return DMA_MAPPING_ERROR;
 
 	dev_addr = xen_phys_to_bus(map);