[SPARK-11251] Fix page size calculation in local mode

```
// My machine only has 8 cores
$ bin/spark-shell --master local[32]
scala> val df = sc.parallelize(Seq((1, 1), (2, 2))).toDF("a", "b")
scala> df.as("x").join(df.as("y"), $"x.a" === $"y.a").count()

Caused by: java.io.IOException: Unable to acquire 2097152 bytes of memory
	at org.apache.spark.util.collection.unsafe.sort.UnsafeExternalSorter.acquireNewPage(UnsafeExternalSorter.java:351)
```

Author: Andrew Or <andrew@databricks.com>

Closes #9209 from andrewor14/fix-local-page-size.