[SPARK-20665][SQL] Bround" and "Round" function return NULL

## What changes were proposed in this pull request?
   spark-sql>select bround(12.3, 2);
   spark-sql>NULL
For this case,  the expected result is 12.3, but it is null.
So ,when the second parameter is bigger than "decimal.scala", the result is not we expected.
"round" function  has the same problem. This PR can solve the problem for both of them.

## How was this patch tested?
unit test cases in MathExpressionsSuite and MathFunctionsSuite

Author: liuxian <liu.xian3@zte.com.cn>

Closes #17906 from 10110346/wip_lx_0509.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/mathExpressions.scala

@@ -1023,10 +1023,10 @@ abstract class RoundBase(child: Expression, scale: Expression,
 
   // not overriding since _scale is a constant int at runtime
   def nullSafeEval(input1: Any): Any = {
-    child.dataType match {
-      case _: DecimalType =>
+    dataType match {
+      case DecimalType.Fixed(_, s) =>
         val decimal = input1.asInstanceOf[Decimal]
-        decimal.toPrecision(decimal.precision, _scale, mode).orNull
+        decimal.toPrecision(decimal.precision, s, mode).orNull
       case ByteType =>
         BigDecimal(input1.asInstanceOf[Byte]).setScale(_scale, mode).toByte
       case ShortType =>
@@ -1055,10 +1055,10 @@ abstract class RoundBase(child: Expression, scale: Expression,
   override def doGenCode(ctx: CodegenContext, ev: ExprCode): ExprCode = {
     val ce = child.genCode(ctx)
 
-    val evaluationCode = child.dataType match {
-      case _: DecimalType =>
+    val evaluationCode = dataType match {
+      case DecimalType.Fixed(_, s) =>
         s"""
-        if (${ce.value}.changePrecision(${ce.value}.precision(), ${_scale},
+        if (${ce.value}.changePrecision(${ce.value}.precision(), ${s},
             java.math.BigDecimal.${modeStr})) {
           ${ev.value} = ${ce.value};
         } else {

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/MathExpressionsSuite.scala

@@ -546,15 +546,14 @@ class MathExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper {
     val bdResults: Seq[BigDecimal] = Seq(BigDecimal(3.0), BigDecimal(3.1), BigDecimal(3.14),
       BigDecimal(3.142), BigDecimal(3.1416), BigDecimal(3.14159),
       BigDecimal(3.141593), BigDecimal(3.1415927))
-    // round_scale > current_scale would result in precision increase
-    // and not allowed by o.a.s.s.types.Decimal.changePrecision, therefore null
+
     (0 to 7).foreach { i =>
       checkEvaluation(Round(bdPi, i), bdResults(i), EmptyRow)
       checkEvaluation(BRound(bdPi, i), bdResults(i), EmptyRow)
     }
     (8 to 10).foreach { scale =>
-      checkEvaluation(Round(bdPi, scale), null, EmptyRow)
-      checkEvaluation(BRound(bdPi, scale), null, EmptyRow)
+      checkEvaluation(Round(bdPi, scale), bdPi, EmptyRow)
+      checkEvaluation(BRound(bdPi, scale), bdPi, EmptyRow)
     }
 
     DataTypeTestUtils.numericTypes.foreach { dataType =>

sql/core/src/test/scala/org/apache/spark/sql/MathFunctionsSuite.scala

@@ -231,6 +231,19 @@ class MathFunctionsSuite extends QueryTest with SharedSQLContext {
       Seq(Row(BigDecimal("0E3"), BigDecimal("0E2"), BigDecimal("0E1"), BigDecimal(3),
         BigDecimal("3.1"), BigDecimal("3.14"), BigDecimal("3.142")))
     )
+
+    val bdPi: BigDecimal = BigDecimal(31415925L, 7)
+    checkAnswer(
+      sql(s"SELECT round($bdPi, 7), round($bdPi, 8), round($bdPi, 9), round($bdPi, 10), " +
+        s"round($bdPi, 100), round($bdPi, 6), round(null, 8)"),
+      Seq(Row(bdPi, bdPi, bdPi, bdPi, bdPi, BigDecimal("3.141593"), null))
+    )
+
+    checkAnswer(
+      sql(s"SELECT bround($bdPi, 7), bround($bdPi, 8), bround($bdPi, 9), bround($bdPi, 10), " +
+        s"bround($bdPi, 100), bround($bdPi, 6), bround(null, 8)"),
+      Seq(Row(bdPi, bdPi, bdPi, bdPi, bdPi, BigDecimal("3.141592"), null))
+    )
   }
 
   test("round/bround with data frame from a local Seq of Product") {