[SPARK-18507][SQL] HiveExternalCatalog.listPartitions should only call getTable once

## What changes were proposed in this pull request?

HiveExternalCatalog.listPartitions should only call `getTable` once, instead of calling it for every partitions.

## How was this patch tested?

N/A

Author: Wenchen Fan <wenchen@databricks.com>

Closes #15978 from cloud-fan/perf.