target: Fix regression bug with handling of zero-length data CDBs

This patch fixes a regression bug with the handling of zero-length
data CDBs within transport_generic_new_cmd() code.  The bug was introduced
with the following commit as part of the single task conversion work:

  commit 4101f0a89d4eb13f04cb0344d59a335b862ca5f9
  Author: Christoph Hellwig <hch@infradead.org>
  Date:   Tue Apr 24 00:25:03 2012 -0400

      target: always allocate a single task

where the zero-length check for SCF_SCSI_DATA_SG_IO_CDB was incorrectly
changed to SCF_SCSI_CONTROL_SG_IO_CDB because of the seperate comment
in transport_generic_new_cmd() wrt to control CDBs zero-length handling
introduced in:

  commit 91ec1d3535b2acf12c599045cc19ad9be3c6a47b
  Author: Nicholas Bellinger <nab@linux-iscsi.org>
  Date:   Fri Jan 13 12:01:34 2012 -0800

      target: Add workaround for zero-length control CDB handling

So go ahead and change transport_generic_new_cmd() to handle control+data
zero-length CDBs in the same manner for this special case.

Tested with iscsi-target + loopback fabric port LUNs on 3.6-rc0 code.

This patch will also need to be picked up for 3.5-stable.

(hch: Add proper comment in transport_generic_new_cmd)

Cc: Christoph Hellwig <hch@lst.de>
Cc: Roland Dreier <roland@purestorage.com>
Cc: Andy Grover <agrover@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_transport.c

@@ -2323,9 +2323,12 @@ int transport_generic_new_cmd(struct se_cmd *cmd)
 		if (ret < 0)
 			goto out_fail;
 	}
-
-	/* Workaround for handling zero-length control CDBs */
-	if (!(cmd->se_cmd_flags & SCF_SCSI_DATA_CDB) && !cmd->data_length) {
+	/*
+	 * If this command doesn't have any payload and we don't have to call
+	 * into the fabric for data transfers, go ahead and complete it right
+	 * away.
+	 */
+	if (!cmd->data_length) {
 		spin_lock_irq(&cmd->t_state_lock);
 		cmd->t_state = TRANSPORT_COMPLETE;
 		cmd->transport_state |= CMD_T_ACTIVE;