usb: usbmon: Read text within supplied buffer size

commit a5f59683 upstream.

This change fixes buffer overflows and silent data corruption with the
usbmon device driver text file read operations.

Signed-off-by: Fredrik Noring <noring@nocrew.org>
Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>