isofs: work-around for Rock Ridge+Joliet CDs with empty ISO root directory

If a CD has both Rock Ridge and Joliet extensions and the ISO root
directory is empty, no files are visible.  Disable Rock Ridge extensions
in this case and use Joliet root directory instead.

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Guenter Roeck <guenter.roeck@ericsson.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>