RDMA/iser: Remove in_interrupt() usage

RDMA/iser: Remove in_interrupt() usage

iser_initialize_task_headers() uses in_interrupt() to find out if it is
safe to acquire a mutex.

in_interrupt() is deprecated as it is ill defined and does not provide
what it suggests. Aside of that it covers only parts of the contexts in
which a mutex may not be acquired.

The following callchains exist:

iscsi_queuecommand() *locks* iscsi_session::frwd_lock
-> iscsi_prep_scsi_cmd_pdu()
   -> session->tt->init_task() (iscsi_iser_task_init())
      -> iser_initialize_task_headers()
-> iscsi_iser_task_xmit() (iscsi_transport::xmit_task)
  -> iscsi_iser_task_xmit_unsol_data()
    -> iser_send_data_out()
      -> iser_initialize_task_headers()

iscsi_data_xmit() *locks* iscsi_session::frwd_lock
-> iscsi_prep_mgmt_task()
   -> session->tt->init_task() (iscsi_iser_task_init())
      -> iser_initialize_task_headers()
-> iscsi_prep_scsi_cmd_pdu()
   -> session->tt->init_task() (iscsi_iser_task_init())
      -> iser_initialize_task_headers()

__iscsi_conn_send_pdu() caller has iscsi_session::frwd_lock
  -> iscsi_prep_mgmt_task()
     -> session->tt->init_task() (iscsi_iser_task_init())
        -> iser_initialize_task_headers()
  -> session->tt->xmit_task() (

The only callchain that is close to be invoked in preemptible context:
iscsi_xmitworker() worker
-> iscsi_data_xmit()
   -> iscsi_xmit_task()
      -> conn->session->tt->xmit_task() (iscsi_iser_task_xmit()

In iscsi_iser_task_xmit() there is this check:
   if (!task->sc)
      return iscsi_iser_mtask_xmit(conn, task);

so it does end up in iser_initialize_task_headers() and
iser_initialize_task_headers() relies on iscsi_task::sc == NULL.

Remove conditional locking of iser_conn::state_mutex because there is no
call chain to do so. Remove the goto label and return early now that there
is no clean up needed.

Link: https://lore.kernel.org/r/20201204174256.62xfcvudndt7oufl@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Sagi Grimberg <sagi@grimberg.me>
Cc: Max Gurtovoy <maxg@nvidia.com>
Cc: Doug Ledford <dledford@redhat.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: linux-rdma@vger.kernel.org
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>