CVE-2026-43408 ceph: add a bunch of missing ceph_path_info initializers

In the Linux kernel, the following vulnerability has been resolved: ceph: add a bunch of missing cephpathinfo initializers cephmdscbuildpath must be called with a zero-initialized cephpathinfo parameter, or else the following cephmdscfreepathinfo may crash. Example crash on Linux 6.18.12:...