RDMA/rtrs: Remove unnecessary argument dir of rtrs_iu_free

The direction of DMA operation is already in the rtrs_iu

Link: https://lore.kernel.org/r/20201023074353.21946-8-jinpu.wang@cloud.ionos.com
Signed-off-by: Gioh Kim <gi-oh.kim@cloud.ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>