Reindex and friends: don't scroll if limit is smaller than scroll

[nik9000]

I wonder if we could avoid adding the scroll to the search that reindex uses if the size of the reindex (or update_by_query or delete_by_query) is less than the scroll size. Or maybe scroll could do this all on its own if there aren't any results after the first set. I'm not sure if it has that information. But I'm fairly sure reindex and fiends could make this call up front.

[elasticmachine]

Pinging @elastic/es-distributed (:Distributed/Reindex)

[henningandersen]

This sounds like a good idea, except for the cases affecting #63671, for instance if conflicts=proceed, we will need to open the scroll regardless, since the expectation is that we will update/delete/reindex max_docs docs if we can.