[SPARK-20280][CORE] FileStatusCache Weigher integer overflow

## What changes were proposed in this pull request?

Weigher.weigh needs to return Int but it is possible for an Array[FileStatus] to have size > Int.maxValue. To avoid this, the size is scaled down by a factor of 32. The maximumWeight of the cache is also scaled down by the same factor.

## How was this patch tested?
New test in FileIndexSuite

Author: Bogdan Raducanu <bogdan@databricks.com>

Closes #17591 from bogdanrdc/SPARK-20280.