ceph-disk: dmcrypt file permissions

The directory in which key files are stored for dmcrypt must be 700 and
the file 600.

http://tracker.ceph.com/issues/9785 Fixes: #9785

Signed-off-by: Loic Dachary <ldachary@redhat.com>
chmod before writing the data to the file?  or, can the file(...) command take a mode?  there is a small window where a fast adversary could open the file as it is created.