Fix overflow check in cudf::merge

[divyegala]

Description

#14250 added a check to ensure cudf::merge throws when the total number of merged rows exceed cudf::size_type limit, however @bdice pointed out that the check was not correct because the accumulation was still occurring in cudf::size_type. This PR computes the accumulation in std::size_t.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[divyegala]

/merge

[ttnghia]

The issue keeps showing up many times and this is just fixing the leaf, not the root of the problem. Probably we should consider a better approach: changing column(_view)::size() to return int64_t instead.