FTS: nsstats num_bytes_used_disk_by_root_reclaimable always reports 0

Build 7.0.0-5127

• Create cluster {kv, index, query} {fts}

  {fts}

• Load `beer-sample` sample bucket
• Create default fts index on `beer-sample` having more than 1 partition and 0 replicas
• check that partitions are distributed through fts nodes
• update all docs in `beer-sample`

update `beer-sample` set state=state||'__'

• check num_bytes_used_disk_by_root_reclaimable stat on all fts nodes , it's always 0.

In case of single fts node this stat shows some positive number greater than 0 after partial or total docs update.