nvmet-tcp: fix a crash in nvmet_req_complete()

in nvmet_tcp_handle_h2c_data_pdu(), if the host sends a data_offset
different from rbytes_done, the driver ends up calling nvmet_req_complete()
passing a status error.
The problem is that at this point cmd->req is not yet initialized,
the kernel will crash after dereferencing a NULL pointer.

Fix the bug by replacing the call to nvmet_req_complete() with
nvmet_tcp_fatal_error().

Fixes: 872d26a ("nvmet-tcp: add NVMe over TCP target driver")
Reviewed-by: Keith Busch <kbsuch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>

Author: maurizio-lombardi

drivers/nvme/target/tcp.c

@@ -998,8 +998,7 @@ static int nvmet_tcp_handle_h2c_data_pdu(struct nvmet_tcp_queue *queue)
 			data->ttag, le32_to_cpu(data->data_offset),
 			cmd->rbytes_done);
 		/* FIXME: use path and transport errors */
-		nvmet_req_complete(&cmd->req,
-			NVME_SC_INVALID_FIELD | NVME_SC_DNR);
+		nvmet_tcp_fatal_error(queue);
 		return -EPROTO;
 	}