ceph-disk: partprobe after creating journal partition

At least one user reports that a partprobe is needed after creating the
journal partition.  It is not clear why sgdisk is not doing it, but this
fixes ceph-disk for them, and should be harmless for other users.

Fixes: #5599
Tested-by: lurbs in #ceph
Signed-off-by: Sage Weil <sage@inktank.com>