Improve vttablet memory consumption with ceph backups

[deepthi]

Reported by @swallowstalker
While taking a backup using ceph storage, vttablet memory consumption goes super high.
Setup details:
vttablet is in one pod with two containers,
200 GB memory for mysqld container
4 GB memory for vttablet container
the database size is 80-ish GB
vttablet memory usage peaks around 9 GB

Backup of the same DB with file storage consumes much less memory (< 500MB)

After upgrading the minio-go library to a newer version in #4598, the memory consumption for a ceph backup decreased somewhat, but is still significantly higher than the file storage mode

[deepthi]

The fix for this might be as simple as detecting the file size and passing it correctly to the minio-go API instead of passing it as -1.
https://github.com/vitessio/vitess/blob/master/go/vt/mysqlctl/cephbackupstorage/ceph.go#L91

For context on similar issues: minio/minio-go#848

[deepthi]

Fixed by #5369