isofs: fix Joliet regression

commit 5404ac8e

 ("isofs: cleanup mount
option processing") missed conversion of joliet option flag resulting
in non-working Joliet support.

CC: walt <w41ter@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>