usb-mtp: outlaw slashes in filenames

usb-mtp: outlaw slashes in filenames

Slash is unix directory separator, so they are not allowed in filenames.
Note this also stops the classic escape via "../".

Fixes: CVE-2018-16867
Reported-by: Michael Hanselmann <public@hansmi.ch>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20181203101045.27976-3-kraxel@redhat.com
(cherry picked from commit c52d46e041b42bb1ee6f692e00a0abe37a9659f6)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>