xprtrdma: Use new CQ API for RPC-over-RDMA client receive CQs

Calling ib_poll_cq() to sort through WCs during a completion is a
common pattern amongst RDMA consumers. Since commit 14d3a3b249
("IB: add a proper completion queue abstraction"), WC sorting can
be handled by the IB core.

By converting to this new API, xprtrdma is made a better neighbor to
other RDMA consumers, as it allows the core to schedule the delivery
of completions more fairly amongst all active consumers.

Because each ib_cqe carries a pointer to a completion method, the
core can now post its own operations on a consumer's QP, and handle
the completions itself, without changes to the consumer.

xprtrdma's reply processing is already handled in a work queue, but
there is some initial order-dependent processing that is done in the
soft IRQ context before a work item is scheduled.

IB_POLL_SOFTIRQ is a direct replacement for the current xprtrdma
receive code path.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Devesh Sharma <devesh.sharma@broadcom.com>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/xprt_rdma.h

@@ -171,6 +171,7 @@ rdmab_to_msg(struct rpcrdma_regbuf *rb)
 struct rpcrdma_buffer;
 
 struct rpcrdma_rep {
+	struct ib_cqe		rr_cqe;
 	unsigned int		rr_len;
 	struct ib_device	*rr_device;
 	struct rpcrdma_xprt	*rr_rxprt;