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 <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Cc: Sagi Grimberg <[email protected]>
Cc: Alex Estrin <[email protected]>
Signed-off-by: Doug Ledford <[email protected]>

Author: Bart Van Assche