rgw: conversion tool to fix broken multipart objects

rgw: conversion tool to fix broken multipart objects

Fixes: #12079
Broken multipart objects: multipart objects that created on 0.94.2
and that start with underscore have a bad locator on their tail objects.
This extends the tool that was needed for older issue we've had with
hammer multipart objects (that start with underscore). The same usage
applies:

$ radosgw-admin bucket check --check-head-obj-locator \
                             --bucket=<bucket> [--fix]

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>