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>
(cherry picked from commit 2af59d5e81c5e3e3d7cfc50d9330d7364659c5eb)