[SPARK-13760][SQL] Fix BigDecimal constructor for FloatType

## What changes were proposed in this pull request?

A very minor change for using `BigDecimal.decimal(f: Float)` instead of `BigDecimal(f: float)`. The latter is deprecated and can result in inconsistencies due to an implicit conversion to `Double`.

## How was this patch tested?

N/A

cc yhuai

Author: Sameer Agarwal <sameer@databricks.com>

Closes #11597 from sameeragarwal/bigdecimal.

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

@@ -748,7 +748,7 @@ case class Round(child: Expression, scale: Expression)
         if (f.isNaN || f.isInfinite) {
           f
         } else {
-          BigDecimal(f).setScale(_scale, HALF_UP).toFloat
+          BigDecimal.decimal(f).setScale(_scale, HALF_UP).toFloat
         }
       case DoubleType =>
         val d = input1.asInstanceOf[Double]