USB: devio: Don't corrupt user memory

The user buffer has "uurb->buffer_length" bytes.  If the kernel has more
information than that, we should truncate it instead of writing past
the end of the user's buffer.  I added a WARN_ONCE() to help the user
debug the issue.

Reported-by: Alan Stern <stern@rowland.harvard.edu>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>