init-ceph: fix osd_data location when checking df utilization

Do not assume default osd data location.

Fixes: #4951
Backport: cuttlefish, bobtail
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Gary Lowelll <gary.lowell@inktank.com>
(cherry picked from commit f2a54cc9c98a9f31aef049c74ea932b2d9000d3c)