iser-target: Work-around live target stack shutdown resource cleanup

iser-target: Work-around live target stack shutdown resource cleanup

Currently we have no way to tell that the target stack is in shutdown
sequence. In case we have open connections, the initiator immediately
attempts to reconnect in a DDOS attack style, so we may end up
terminating the iser enabled network portal while it's np_accept_list
still have pending connections.

The workaround is simply release all the connections in the list.
A proper fix will be to start shutdown sequence by shutting the
network portal to avoid initiator immediate reconnect attempts.

But the temporary work around seems to work at this point, so I think
we can do this for now...

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