nvmet-tcp: fix possible list corruption for unexpected command failure

nvmet_tcp_handle_req_failure needs to understand weather to prepare
for incoming data or the next pdu. However if we misidentify this, we
will wait for 0-length data, and queue the response although nvmet_req_init
already did that.

The particular command was namespace management command with no data,
which was incorrectly categorized as a command with incapsule data.

Also, add a code comment of what we are trying to do here.

Signed-off-by: Sagi Grimberg <[e-mail]>
Reviewed-by: Keith Busch <[e-mail]>
Signed-off-by: Christoph Hellwig <[e-mail]>

drivers/nvme/target/tcp.c

···
 	size_t data_len = le32_to_cpu(req->cmd->common.dptr.sgl.length);
 	int ret;
 
-	if (!nvme_is_write(cmd->req.cmd) ||
+	/*
+	 * This command has not been processed yet, hence we are trying to
+	 * figure out if there is still pending data left to receive. If
+	 * we don't, we can simply prepare for the next pdu and bail out,
+	 * otherwise we will need to prepare a buffer and receive the
+	 * stale data before continuing forward.
+	 */
+	if (!nvme_is_write(cmd->req.cmd) || !data_len ||
 	    data_len > cmd->req.port->inline_data_size) {
 		nvmet_prepare_receive_pdu(queue);
 		return;