RDMA/nes: Update refcnt during disconnect

During termination, it is possible for the refcnt to go to zero while
the worker thread is posting events upward.  This fix increments the
refcnt before the request is passed to the worker thread.  The thread
decrements the refcnt when the request is completed.

Signed-off-by: Don Wood <donald.e.wood@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>