qla2xxx: delay plogi/prli ack until existing sessions are deleted

- keep qla_tgt_sess object on the session list until it's freed

- modify use of sess->deleted flag to differentiate delayed
  session deletion that can be cancelled from irreversible one:
  QLA_SESS_DELETION_PENDING vs QLA_SESS_DELETION_IN_PROGRESS

- during IN_PROGRESS deletion all newly arrived commands and TMRs will
  be rejected, existing commands and TMRs will be terminated when
  given by the core to the fabric or simply dropped if session logout
  has already happened (logout terminates all existing exchanges)

- new PLOGI will initiate deletion of the following sessions
  (unless deletion is already IN_PROGRESS):
  - with the same port_name (with logout)
  - different port_name, different loop_id but the same port_id
    (with logout)
  - different port_name, different port_id, but the same loop_id
    (without logout)

- additionally each new PLOGI will store imm notify iocb in the
  same port_name session being deleted. When deletion process
  completes this iocb will be acked. Only the most recent PLOGI
  iocb is stored. The older ones will be terminated when replaced.

- new PRLI will initiate deletion of the following sessions
  (unless deletion is already IN_PROGRESS):
  - different port_name, different port_id, but the same loop_id
   (without logout)

Cc: <stable@vger.kernel.org> # v3.18+
Signed-off-by: Alexei Potashnik <alexei@purestorage.com>
Acked-by: Quinn Tran <quinn.tran@qlogic.com>
Signed-off-by: Himanshu Madhani <himanshu.madhani@qlogic.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>