nouveau: reset dma_nr in nouveau_dmem_migrate_alloc_and_copy

When we start a new batch of dma_map operations we need to reset dma_nr,
as we start filling a newly allocated array.

Link: https://lore.kernel.org/r/20190814075928.23766-3-hch@lst.deSigned-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Tested-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/gpu/drm/nouveau/nouveau_dmem.c

@@ -682,6 +682,7 @@ nouveau_dmem_migrate_alloc_and_copy(struct vm_area_struct *vma,
 	migrate->dma = kmalloc(sizeof(*migrate->dma) * npages, GFP_KERNEL);
 	if (!migrate->dma)
 		goto error;
+	migrate->dma_nr = 0;
 
 	/* Copy things over */
 	copy = drm->dmem->migrate.copy_func;