i386: make pci_map_single/pci_map_sg warn for zero length.

As suggested by Linus.

This catches driver bugs that could cause corruption on IOMMU architectures.

Also I converted the BUGs to out_of_line_bug()s to save a bit 
of text space.

Signed-off-by: Andi Kleen <ak@suse.de>

Index: linux/include/asm-i386/dma-mapping.h
===================================================================
--- linux.orig/include/asm-i386/dma-mapping.h
+++ linux/include/asm-i386/dma-mapping.h
@@ -6,6 +6,7 @@
 #include <asm/cache.h>
 #include <asm/io.h>
 #include <asm/scatterlist.h>
+#include <asm/bug.h>
 
 #define dma_alloc_noncoherent(d, s, h, f) dma_alloc_coherent(d, s, h, f)
 #define dma_free_noncoherent(d, s, v, h) dma_free_coherent(d, s, v, h)
@@ -20,7 +21,9 @@ static inline dma_addr_t
 dma_map_single(struct device *dev, void *ptr, size_t size,
 	       enum dma_data_direction direction)
 {
-	BUG_ON(direction == DMA_NONE);
+	if (direction == DMA_NONE)
+		BUG();
+	WARN_ON(size == 0);
 	flush_write_buffers();
 	return virt_to_phys(ptr);
 }
@@ -29,7 +32,8 @@ static inline void
 dma_unmap_single(struct device *dev, dma_addr_t dma_addr, size_t size,
 		 enum dma_data_direction direction)
 {
-	BUG_ON(direction == DMA_NONE);
+	if (direction == DMA_NONE)
+		BUG();
 }
 
 static inline int
@@ -38,7 +42,9 @@ dma_map_sg(struct device *dev, struct sc
 {
 	int i;
 
-	BUG_ON(direction == DMA_NONE);
+	if (direction == DMA_NONE)
+		BUG();
+	WARN_ON(nents == 0 || sg[0].length == 0);
 
 	for (i = 0; i < nents; i++ ) {
 		BUG_ON(!sg[i].page);