RDMA/rxe: Implement flush completion

Per IBA SPEC, FLUSH will ack in rdma read response with 0 length.

Use IB_WC_FLUSH (aka IB_UVERBS_WC_FLUSH) code to tell userspace a FLUSH
completion.

Link: https://lore.kernel.org/r/20221206130201.30986-9-lizhijian@fujitsu.comReviewed-by: NZhu Yanjun <zyjzyj2000@gmail.com>
Signed-off-by: NLi Zhijian <lizhijian@fujitsu.com>
Signed-off-by: NJason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/sw/rxe/rxe_comp.c

@@ -105,6 +105,7 @@ static enum ib_wc_opcode wr_to_wc_opcode(enum ib_wr_opcode opcode)
 	case IB_WR_REG_MR:			return IB_WC_REG_MR;
 	case IB_WR_BIND_MW:			return IB_WC_BIND_MW;
 	case IB_WR_ATOMIC_WRITE:		return IB_WC_ATOMIC_WRITE;
+	case IB_WR_FLUSH:			return IB_WC_FLUSH;
 
 	default:
 		return 0xff;
@@ -278,7 +279,8 @@ static inline enum comp_state check_ack(struct rxe_qp *qp,
 		 */
 	case IB_OPCODE_RC_RDMA_READ_RESPONSE_MIDDLE:
 		if (wqe->wr.opcode != IB_WR_RDMA_READ &&
-		    wqe->wr.opcode != IB_WR_RDMA_READ_WITH_INV) {
+		    wqe->wr.opcode != IB_WR_RDMA_READ_WITH_INV &&
+		    wqe->wr.opcode != IB_WR_FLUSH) {
 			wqe->status = IB_WC_FATAL_ERR;
 			return COMPST_ERROR;
 		}