pktgen: correctly handle failures when adding a device

The return value of pktgen_add_device() is not checked, so
even if we fail to add some device, for example, non-exist one,
we still see "OK:...". This patch fixes it.

After this patch, I got:

	# echo "add_device non-exist" > /proc/net/pktgen/kpktgend_0
	-bash: echo: write error: No such device
	# cat /proc/net/pktgen/kpktgend_0
	Running:
	Stopped:
	Result: ERROR: can not add device non-exist
	# echo "add_device eth0" > /proc/net/pktgen/kpktgend_0
	# cat /proc/net/pktgen/kpktgend_0
	Running:
	Stopped: eth0
	Result: OK: add_device=eth0

(Candidate for -stable)

Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/pktgen.c

@@ -1781,10 +1781,13 @@ static ssize_t pktgen_thread_write(struct file *file,
 			return -EFAULT;
 		i += len;
 		mutex_lock(&pktgen_thread_lock);
-		pktgen_add_device(t, f);
+		ret = pktgen_add_device(t, f);
 		mutex_unlock(&pktgen_thread_lock);
+		if (!ret) {
 			ret = count;
 			sprintf(pg_result, "OK: add_device=%s", f);
+		} else
+			sprintf(pg_result, "ERROR: can not add device %s", f);
 		goto out;
 	}