orangefs: clean up fill_default_sys_attrs

Size and type are read-only and not in the mask. The times were left
unset despite being in the mask.

We zero-fill the times since the server will fill them in and we will
get the correct time when we fill the inode with getattr.

Signed-off-by: Martin Brandenburg <martin@omnibond.com>
Signed-off-by: Mike Marshall <hubcap@omnibond.com>