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>
(cherry picked from commit 0f196265f049d432e399197a3af3f90d2e916275)