ceph-disk: partprobe before settle when preparing dev

Two users have reported this fixes a problem with using --dmcrypt.

Fixes: #6966
Tested-by: Eric Eastman <eric0e@aol.com>
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1777 from ceph/wip-6966

ceph-disk: partprobe before settle when preparing dev

Reviewed-by: Alfredo Deza <alfredo.deza@inktank.com>