[SPARK-21312][SQL] correct offsetInBytes in UnsafeRow.writeToStream

## What changes were proposed in this pull request?

Corrects offsetInBytes calculation in UnsafeRow.writeToStream. Known failures include writes to some DataSources that have own SparkPlan implementations and cause EXCHANGE in writes.

## How was this patch tested?

Extended UnsafeRowSuite.writeToStream to include an UnsafeRow over byte array having non-zero offset.

Author: Sumedh Wale <swale@snappydata.io>

Closes #18535 from sumwale/SPARK-21312.

(cherry picked from commit 14a3bb3a)
Signed-off-by: Wenchen Fan <wenchen@databricks.com>