USB: usb-skeleton.c: fix compilation error and restored kref_put on fail in skel_open

Fixing compilaton error.
Incrementing usage counter only on successful execution of skel_open.
Removing redundant locking

Some last changes in function skel_open and finally commit
52a74999 Revert "USB: usb-skeleton.c: fix open/disconnect race"
introduced a bug in function skel_open, which this patch fixes.

Changes since v2:
- refactoring
- Removing redundant mutex synchronization.

Changes since v1:
- Fixed accordingly feedback of Oliver Neukum oneukum@suse.de: also need to drop the lock.
Signed-off-by: NConstantine Shulyupin <const@MakeLinux.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/usb-skeleton.c

@@ -105,20 +105,15 @@ static int skel_open(struct inode *inode, struct file *file)
 		goto exit;
 	}
 
-	/* increment our usage count for the device */
-	kref_get(&dev->kref);
-
-	/* lock the device to allow correctly handling errors
-	 * in resumption */
-	mutex_lock(&dev->io_mutex);
-
 	retval = usb_autopm_get_interface(interface);
 	if (retval)
-		goto out_err;
+		goto exit;
+
+	/* increment our usage count for the device */
+	kref_get(&dev->kref);
 
 	/* save our object in the file's private structure */
 	file->private_data = dev;
-	mutex_unlock(&dev->io_mutex);
 
 exit:
 	return retval;