swiotlb: fix wrong panic

swiotlb's map_page wrongly calls panic() when it can't find a buffer fit
for device's dma mask.  It should return an error instead.

Devices with an odd dma mask (i.e.  under 4G) like b44 network card hit
this bug (the system crashes):

   http://marc.info/?l=linux-kernel&m=129648943830106&w=2

If swiotlb returns an error, b44 driver can use the own bouncing
mechanism.
Reported-by: NChuck Ebbert <cebbert@redhat.com>
Signed-off-by: NFUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Tested-by: NArkadiusz Miskiewicz <arekm@maven.pl>
Cc: <stable@kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

lib/swiotlb.c

@@ -686,8 +686,10 @@ dma_addr_t swiotlb_map_page(struct device *dev, struct page *page,
 	/*
 	 * Ensure that the address returned is DMA'ble
 	 */
-	if (!dma_capable(dev, dev_addr, size))
-		panic("map_single: bounce buffer is not DMA'ble");
+	if (!dma_capable(dev, dev_addr, size)) {
+		swiotlb_tbl_unmap_single(dev, map, size, dir);
+		dev_addr = swiotlb_virt_to_bus(dev, io_tlb_overflow_buffer);
+	}
 
 	return dev_addr;
 }