ceph: only link open operations to directory unsafe list if O_CREAT|O_TRUNC

We only need to put these on the directory unsafe list if they have
side effects that fsync(2) should flush out.

Reviewed-by: Yehuda Sadeh <yehuda@hq.newdream.net>
Signed-off-by: Sage Weil <sage@newdream.net>