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>