net/9p: Handle mount errors correctly.

With this patch we have

# mount -t 9p -o trans=virtio virtio2 /mnt/
# mount -t 9p -o trans=virtio virtio2 /mnt/
mount: virtio2 already mounted or /mnt/ busy
mount: according to mtab, virtio2 is already mounted on /mnt
# mount -t 9p -o trans=virtio virtio3 /mnt/ -o debug=0xfff
mount: special device virtio3 does not exist

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

net/9p/trans_virtio.c

@@ -279,6 +279,7 @@ static int
 p9_virtio_create(struct p9_client *client, const char *devname, char *args)
 {
 	struct virtio_chan *chan;
+	int ret = -ENOENT;
 	int found = 0;
 
 	mutex_lock(&virtio_9p_lock);
@@ -289,13 +290,14 @@ p9_virtio_create(struct p9_client *client, const char *devname, char *args)
 				found = 1;
 				break;
 			}
+			ret = -EBUSY;
 		}
 	}
 	mutex_unlock(&virtio_9p_lock);
 
 	if (!found) {
 		printk(KERN_ERR "9p: no channels available\n");
-		return -ENODEV;
+		return ret;
 	}
 
 	client->trans = (void *)chan;