iscsi-target: Change BUG_ON to REJECT in iscsit_process_nop_out

This patch changes an incorrect use of BUG_ON to instead generate a
REJECT + PROTOCOL_ERROR in iscsit_process_nop_out() code.  This case
can occur with traditional TCP where a flood of zeros in the data
stream can reach this block for what is presumed to be a NOP-OUT with
a solicited reply, but without a valid iscsi_cmd pointer.

This incorrect BUG_ON was introduced during the v3.11-rc timeframe
with the following commit:

commit 778de368
Author: Nicholas Bellinger <nab@linux-iscsi.org>
Date:   Fri Jun 14 16:07:47 2013 -0700

    iscsi/isert-target: Refactor ISCSI_OP_NOOP RX handling
Reported-by: NArshad Hussain <arshad.hussain@calsoftinc.com>
Cc: stable@vger.kernel.org # 3.11+
Signed-off-by: NNicholas Bellinger <nab@linux-iscsi.org>

drivers/target/iscsi/iscsi_target.c

@@ -1593,7 +1593,9 @@ int iscsit_process_nop_out(struct iscsi_conn *conn, struct iscsi_cmd *cmd,
 	 * Initiator is expecting a NopIN ping reply..
 	 */
 	if (hdr->itt != RESERVED_ITT) {
-		BUG_ON(!cmd);
+		if (!cmd)
+			return iscsit_add_reject(conn, ISCSI_REASON_PROTOCOL_ERROR,
+						(unsigned char *)hdr);
 
 		spin_lock_bh(&conn->cmd_lock);
 		list_add_tail(&cmd->i_conn_node, &conn->conn_cmd_list);