dma-mapping: remove the DMA_ATTR_WRITE_BARRIER flag

This flag is not implemented by any backend and only set by the ib_umem
module in a single instance.

Link: https://lore.kernel.org/r/20191113073214.9514-2-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>