RDMA/cxgb3: Don't free endpoints early

- Keep ref on connection request endpoints until either accepted or
  rejected so it doesn't get freed early.

- Endpoint flags now need to be set via atomic bitops because they can
  be set on both the iw_cxgb3 workqueue thread and user disconnect
  threads.

- Don't move out of CLOSING too early due to multiple calls to
  iwch_ep_disconnect.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>