iommu: Do not return error code for APIs with size_t return type

Currently, iommu_unmap, iommu_unmap_fast and iommu_map_sg return
size_t.  However, some of the return values are error codes (< 0),
which can be misinterpreted as large size. Therefore, returning size 0
instead to signify failure to map/unmap.

Cc: Joerg Roedel <joro@8bytes.org>
Cc: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>
diff --git a/include/linux/iommu.h b/include/linux/iommu.h
index 41b8c57..19938ee 100644
--- a/include/linux/iommu.h
+++ b/include/linux/iommu.h
@@ -465,23 +465,23 @@ static inline int iommu_map(struct iommu_domain *domain, unsigned long iova,
 	return -ENODEV;
 }
 
-static inline int iommu_unmap(struct iommu_domain *domain, unsigned long iova,
-			      size_t size)
+static inline size_t iommu_unmap(struct iommu_domain *domain,
+				 unsigned long iova, size_t size)
 {
-	return -ENODEV;
+	return 0;
 }
 
-static inline int iommu_unmap_fast(struct iommu_domain *domain, unsigned long iova,
-				   int gfp_order)
+static inline size_t iommu_unmap_fast(struct iommu_domain *domain,
+				      unsigned long iova, int gfp_order)
 {
-	return -ENODEV;
+	return 0;
 }
 
 static inline size_t iommu_map_sg(struct iommu_domain *domain,
 				  unsigned long iova, struct scatterlist *sg,
 				  unsigned int nents, int prot)
 {
-	return -ENODEV;
+	return 0;
 }
 
 static inline void iommu_flush_tlb_all(struct iommu_domain *domain)