usbcore: cdev lock_kernel() pushdown

usb_open() is protected by a down_read(&minor_rwsem), but I'm not sure I
trust it to protect everything including subsidiary open() functions.
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

drivers/usb/core/file.c

@@ -18,6 +18,7 @@
 #include <linux/module.h>
 #include <linux/errno.h>
 #include <linux/rwsem.h>
+#include <linux/smp_lock.h>
 #include <linux/usb.h>
 
 #include "usb.h"
@@ -33,6 +34,7 @@ static int usb_open(struct inode * inode, struct file * file)
 	int err = -ENODEV;
 	const struct file_operations *old_fops, *new_fops = NULL;
 
+	lock_kernel();
 	down_read(&minor_rwsem);
 	c = usb_minors[minor];
 
@@ -51,6 +53,7 @@ static int usb_open(struct inode * inode, struct file * file)
 	fops_put(old_fops);
  done:
 	up_read(&minor_rwsem);
+	unlock_kernel();
 	return err;
 }