usbdev: BKL pushdown

Add explicit lock_kernel() calls to usbdev_open()

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/drivers/usb/core/devio.c b/drivers/usb/core/devio.c
index de17738..9218cca 100644
--- a/drivers/usb/core/devio.c
+++ b/drivers/usb/core/devio.c
@@ -565,6 +565,7 @@
 	struct dev_state *ps;
 	int ret;
 
+	lock_kernel();
 	/* Protect against simultaneous removal or release */
 	mutex_lock(&usbfs_mutex);
 
@@ -611,6 +612,7 @@
 	if (ret)
 		kfree(ps);
 	mutex_unlock(&usbfs_mutex);
+	unlock_kernel();
 	return ret;
 }