examples/filesystem: posix open needs mode if O_CREATE
examples/filesystem: posix open needs mode if O_CREATE

umask is applied to mode (mode & ~umask) -> 00777 is a good default
see: man 2 open

kYc0o/RIOT,miri64/RIOT,kaspar030/RIOT,ant9000/RIOT,jasonatran/RIOT,jasonatran/RIOT,ant9000/RIOT,OlegHahm/RIOT,authmillenon/RIOT,OTAkeys/RIOT,kaspar030/RIOT,kaspar030/RIOT,kaspar030/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,authmillenon/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,miri64/RIOT,ant9000/RIOT,OTAkeys/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,ant9000/RIOT,miri64/RIOT,jasonatran/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,kaspar030/RIOT,kYc0o/RIOT,authmillenon/RIOT,miri64/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,authmillenon/RIOT,miri64/RIOT,OlegHahm/RIOT,ant9000/RIOT,authmillenon/RIOT,jasonatran/RIOT