iommufd/iova_bitmap: Move initial pinning to iova_bitmap_for_each()

The pinned pages are only relevant when it starts iterating the bitmap so
defer that into iova_bitmap_for_each().

Link: https://lore.kernel.org/r/20240627110105.62325-9-joao.m.martins@oracle.com
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Tested-by: Matt Ochs <mochs@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/iommu/iommufd/iova_bitmap.c

@@ -269,9 +269,6 @@ struct iova_bitmap *iova_bitmap_alloc(unsigned long iova, size_t length,
 		goto err;
 	}
 
-	rc = iova_bitmap_get(bitmap);
-	if (rc)
-		goto err;
 	return bitmap;
 
 err:
@@ -425,6 +422,10 @@ int iova_bitmap_for_each(struct iova_bitmap *bitmap, void *opaque,
 {
 	int ret = 0;
 
+	ret = iova_bitmap_get(bitmap);
+	if (ret)
+		return ret;
+
 	for (; !iova_bitmap_done(bitmap) && !ret;
 	     ret = iova_bitmap_advance(bitmap)) {
 		ret = fn(bitmap, iova_bitmap_mapped_iova(bitmap),