tools/cephfs: add basic detection/cleanup tool for dentry first damage

tools/cephfs: add basic detection/cleanup tool for dentry first damage

This tool iterates through the metadata pool looking for dentries with
non-sensical "first" (snapshot) metadata. It only specifically looks for
damage of the kind where the "first" value is greater than the latest
generated snapshot for the file system. It does not detect other kinds
of genuine damage to the "first" metadata.

Fixes: https://tracker.ceph.com/issues/56140
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>