target: Don't call TFO->write_pending if data_length == 0

commit 885e7b0e upstream.

If an initiator sends a zero-length command (e.g. TEST UNIT READY) but
sets the transfer direction in the transport layer to indicate a
data-out phase, we still shouldn't try to transfer data.  At best it's
a NOP, and depending on the transport, we might crash on an
uninitialized sg list.
Reported-by: Craig Watson <craig.watson@vanguard-rugged.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/target/target_core_transport.c

@@ -2295,7 +2295,7 @@ transport_generic_new_cmd(struct se_cmd *cmd)
 	 * and let it call back once the write buffers are ready.
 	 */
 	target_add_to_state_list(cmd);
-	if (cmd->data_direction != DMA_TO_DEVICE) {
+	if (cmd->data_direction != DMA_TO_DEVICE || cmd->data_length == 0) {
 		target_execute_cmd(cmd);
 		return 0;
 	}