iscsi-target: Fix ImmediateData=Yes failure regression in >= v3.10

iscsi-target: Fix ImmediateData=Yes failure regression in >= v3.10

This patch addresses a regression bug within ImmediateData=Yes failure
handling that ends up triggering an OOPs within >= v3.10 iscsi-target
code.

The problem occurs when iscsit_process_scsi_cmd() does the call to
target_put_sess_cmd(), and once again in iscsit_get_immediate_data()
that is triggered during two different cases:

 - When iscsit_sequence_cmd() returns CMDSN_LOWER_THAN_EXP, for which
   the descriptor state will already have been set to ISTATE_REMOVE
   by iscsit_sequence_cmd(), and
 - When iscsi_cmd->sense_reason is set, for which iscsit_execute_cmd()
   will have already called transport_send_check_condition_and_sense()
   to queue the exception response.

It changes iscsit_process_scsi_cmd() to drop the early call, and makes
iscsit_get_immediate_data() call target_put_sess_cmd() from a single
location after dumping the immediate data for the failed command.

The regression was initially introduced in commit:

commit 561bf15892375597ee59d473a704a3e634c4f311
Author: Nicholas Bellinger <nab@linux-iscsi.org>
Date:   Wed Jul 3 03:58:58 2013 -0700

    iscsi-target: Fix iscsit_sequence_cmd reject handling for iser

Cc: stable@vger.kernel.org # 3.10+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>