RDMA/cxgb3: Correctly serialize peer abort path

Open MPI and other stress testing exposed a few bad bugs in handling
aborts in the middle of a normal close.  Fix these by:

 - serializing abort reply and peer abort processing with disconnect
   processing

 - warning (and ignoring) if ep timer is stopped when it wasn't running

 - cleaning up disconnect path to correctly deal with aborting and
   dead endpoints

 - in iwch_modify_qp(), taking a ref on the ep before releasing the qp
   lock if iwch_ep_disconnect() will be called.  The ref is dropped
   after calling disconnect.

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