ceph.spec.in: use %{_sbindir} macro again

Commit ae71b576a7396bf44f052845af7561cc0436486a change some paths
again to absolute path to /usr/sbin. Use the correct macro %{_sbindir}
again as done in 4b90a958d11059bf470897cbedf460cd9c159dd6.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>