confusing low threads error for import into

## Enhancement

When performing `import into` with global sort on a 4C dedicated cluster, it reports an error `ERROR 8173 (HY000): PreCheck failed: global sort requires at least 8 threads`. But actually we specify `thread=16` on the `import into`statement. Seems no matter what the `thread` argument you specify, it rounds to the maximum allow value(cpu core number). I think this is a bit confusing and might throw an error or at least warning when user specify a thread value larger than the maximum allow value.