RDMA/rxe: Check rxe_get() return value

In the tasklets (completer, responder, and requester) check the return
value from rxe_get() to detect failures to get a reference.  This only
occurs if the qp has had its reference count drop to zero which indicates
that it no longer should be used.

The ref is never 0 today because the tasklets are flushed before the ref
is dropped. The next patch changes this so that the ref is dropped then
the tasklets are flushed.

Link: https://lore.kernel.org/r/20220421014042.26985-4-rpearsonhpe@gmail.com
Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/sw/rxe/rxe_comp.c

@@ -562,7 +562,8 @@ int rxe_completer(void *arg)
 	enum comp_state state;
 	int ret = 0;
 
-	rxe_get(qp);
+	if (!rxe_get(qp))
+		return -EAGAIN;
 
 	if (!qp->valid || qp->req.state == QP_STATE_ERROR ||
 	    qp->req.state == QP_STATE_RESET) {

drivers/infiniband/sw/rxe/rxe_req.c

@@ -606,7 +606,8 @@ int rxe_requester(void *arg)
 	struct rxe_ah *ah;
 	struct rxe_av *av;
 
-	rxe_get(qp);
+	if (!rxe_get(qp))
+		return -EAGAIN;
 
 next_wqe:
 	if (unlikely(!qp->valid || qp->req.state == QP_STATE_ERROR))

drivers/infiniband/sw/rxe/rxe_resp.c

@@ -1247,7 +1247,8 @@ int rxe_responder(void *arg)
 	struct rxe_pkt_info *pkt = NULL;
 	int ret = 0;
 
-	rxe_get(qp);
+	if (!rxe_get(qp))
+		return -EAGAIN;
 
 	qp->resp.aeth_syndrome = AETH_ACK_UNLIMITED;