dma-mapping: Convert NO_DMA get_dma_ops() into a real dummy

If NO_DMA=y, get_dma_ops() returns a reference to the
non-existing symbol bad_dma_ops, thus causing a link failure if it is
ever used.

Make get_dma_ops() return NULL instead, to avoid the link failure.
This allows to improve compile-testing, and limits the need to keep on
sprinkling dependencies on HAS_DMA all over the place.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Acked-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

include/linux/dma-mapping.h

@@ -212,14 +212,14 @@ static inline void set_dma_ops(struct device *dev,
 }
 #else
 /*
- * Define the dma api to allow compilation but not linking of
- * dma dependent code.  Code that depends on the dma-mapping
- * API needs to set 'depends on HAS_DMA' in its Kconfig
+ * Define the dma api to allow compilation of dma dependent code.
+ * Code that depends on the dma-mapping API needs to set 'depends on HAS_DMA'
+ * in its Kconfig, unless it already depends on <something> || COMPILE_TEST,
+ * where <something> guarantuees the availability of the dma-mapping API.
  */
-extern const struct dma_map_ops bad_dma_ops;
 static inline const struct dma_map_ops *get_dma_ops(struct device *dev)
 {
-	return &bad_dma_ops;
+	return NULL;
 }
 #endif