IB/srpt: Detect session shutdown reliably

The Last WQE Reached event is only generated after one or more work
requests have been queued on the QP associated with a session. Since
session shutdown can start before any work requests have been queued,
use a zero-length RDMA write to wait until a QP has been drained.

Additionally, rework the code for closing and disconnecting a session.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Sagi Grimberg <sagig@mellanox.com>
Cc: Alex Estrin <alex.estrin@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>