iser-target: Fix logout sequence

We don't want to wait for conn_logout_comp from isert_comp_wq
context as this blocks further completions from being processed.
Instead we wait for it conditionally (if logout response was
actually posted) in wait_conn. This wait should normally happen
immediately as it occurs after we consumed all the completions
(including flush errors) and conn_logout_comp should have been
completed.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>