fix confusing error report of connect

[jjqq2013]

There is a confusing error message when failed to connect to tgtd unix domain socket, it could not check the error,
instead, it just continue to call write, which in turn cause error failed to send request hdr to tgt daemon, Transport endpoint is not connected.

	err = ipc_mgmt_connect(&fd);
	if (err < 0) {

When ipc_mgmt_connect failed, it returns errno as err, so the err will be a positive value, so when error happens, this check does not works.

		eprintf("can't connect to tgt daemon, %m\n");
		goto out;
	}

	err = write(fd, req, sizeof(*req));

In fact, it should report error when connect: can't connect to tgt daemon, THE REAL REASON