iommu/dma: Cleanup dma-iommu.h

No need for a __KERNEL__ guard outside uapi and add a missing comment
describing the #else cpp statement.  Last but not least include
<linux/errno.h> instead of the asm version, which is frowned upon.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

include/linux/dma-iommu.h

@@ -16,9 +16,8 @@
 #ifndef __DMA_IOMMU_H
 #define __DMA_IOMMU_H
 
-#ifdef __KERNEL__
+#include <linux/errno.h>
 #include <linux/types.h>
-#include <asm/errno.h>
 
 #ifdef CONFIG_IOMMU_DMA
 #include <linux/dma-mapping.h>
@@ -86,7 +85,7 @@ void iommu_dma_compose_msi_msg(struct msi_desc *desc,
 
 void iommu_dma_get_resv_regions(struct device *dev, struct list_head *list);
 
-#else
+#else /* CONFIG_IOMMU_DMA */
 
 struct iommu_domain;
 struct msi_desc;
@@ -128,5 +127,4 @@ static inline void iommu_dma_get_resv_regions(struct device *dev, struct list_he
 }
 
 #endif	/* CONFIG_IOMMU_DMA */
-#endif	/* __KERNEL__ */
 #endif	/* __DMA_IOMMU_H */