[SPARK-20868][CORE] UnsafeShuffleWriter should verify the position after FileChannel.transferTo

## What changes were proposed in this pull request?

Long time ago we fixed a [bug](https://issues.apache.org/jira/browse/SPARK-3948) in shuffle writer about `FileChannel.transferTo`. We were not very confident about that fix, so we added a position check after the writing, try to discover the bug earlier.

 However this checking is missing in the new `UnsafeShuffleWriter`, this PR adds it.

https://issues.apache.org/jira/browse/SPARK-18105

 maybe related to that `FileChannel.transferTo` bug, hopefully we can find out the root cause after adding this position check.

## How was this patch tested?

N/A

Author: Wenchen Fan <wenchen@databricks.com>

Closes #18091 from cloud-fan/shuffle.

(cherry picked from commit d9ad7890)
Signed-off-by: Wenchen Fan <wenchen@databricks.com>