[SPARK-3935][Core] log the number of records that has been written

There is a unused variable(count) in saveAsHadoopDataset in PairRDDFunctions.scala. The initial idea of this variable seems to count the number of records, so I am adding a log statement to log the number of records that has been written to the writer.

Author: likun <jacky.likun@huawei.com>
Author: jackylk <jacky.likun@huawei.com>

Closes #2791 from jackylk/SPARK-3935 and squashes the following commits:

a874047 [jackylk] removing the unused variable in PairRddFunctions.scala
3bf43c7 [likun] log the number of records has been written