RDMA/cxgb4: Make sure flush CQ entries are collected on connection close

At the time when a peer closes the connection, iw_cxgb4 will not send
a cq event if ibqp.uobject exists.  In that case, its possible for a
user application to get blocked in ibv_get_cq_event().

To resolve this, call the cq's comp_handler to unblock any read from
ibv_get_cq_event().  This will trigger userspace to poll the cq and
collect flush status completions for any pending work requests.

Signed-off-by: Kumar Sanghvi <kumaras@chelsio.com>
Acked-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>