RDMA/rtrs: Introduce rtrs_post_send

Since the three functions share the similar logic, let's introduce one
common function for it.

Link: https://lore.kernel.org/r/20201023074353.21946-12-jinpu.wang@cloud.ionos.com
Signed-off-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>