RDMA/nes: Rework the disconn routine for terminate and flushing

The disconn routine has been reworked to acoomodate the terminate and
flushing changes.  The routine has been reorganized to make all the
decisions at the start then it performs all the required operations.
This simplified the lock handling and is easier to follow.

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