[SPARK-8841] [SQL] Fix partition pruning percentage log message

When pruning partitions for a query plan, a message is logged indicating what how many partitions were selected based on predicate criteria, and what percent were pruned.

The current release erroneously uses `1 - total/selected` to compute this quantity, leading to nonsense messages like "pruned -1000% partitions". The fix is simple and obvious.

Author: Steve Lindemann <steve.lindemann@engineersgatelp.com>

Closes #7227 from srlindemann/master and squashes the following commits:

c788061 [Steve Lindemann] fix percentPruned log message

sql/core/src/main/scala/org/apache/spark/sql/sources/DataSourceStrategy.scala

@@ -65,7 +65,7 @@ private[sql] object DataSourceStrategy extends Strategy with Logging {
       logInfo {
         val total = t.partitionSpec.partitions.length
         val selected = selectedPartitions.length
-        val percentPruned = (1 - total.toDouble / selected.toDouble) * 100
+        val percentPruned = (1 - selected.toDouble / total.toDouble) * 100
         s"Selected $selected partitions out of $total, pruned $percentPruned% partitions."
       }