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

Calling ib_poll_cq() to sort through WCs during a completion is a
common pattern amongst RDMA consumers. Since commit 14d3a3b2
("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.

Send completions were previously handled entirely in the completion
upcall handler (ie, deferring to a process context is unneeded).
Thus IB_POLL_SOFTIRQ is a direct replacement for the current
xprtrdma send 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>