iser-target: release stale iser connections

When receiving a new iser connect request we serialize
the pending requests by adding the newly created iser connection
to the np accept list and let the login thread process the connect
request one by one (np_accept_wait).

In case we received a disconnect request before the iser_conn
has begun processing (still linked in np_accept_list) we should
detach it from the list and clean it up and not have the login
thread process a stale connection. We do it only when the connection
state is not already terminating (initiator driven disconnect) as
this might lead us to access np_accept_mutex after the np was released
in live shutdown scenarios.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Jenny Falkovich <jennyf@mellanox.com>
Cc: stable@vger.kernel.org # 3.10+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>