[SPARK-20386][SPARK CORE] modify the log info if the block exists on the slave already

## What changes were proposed in this pull request?
Modify the added memory size to memSize-originalMemSize if the  block exists on the slave already
since if the  block exists, the added memory size should be memSize-originalMemSize; if originalMemSize is bigger than memSize ,then the log info should be Removed memory, removed size should be originalMemSize-memSize

## How was this patch tested?
Multiple runs on existing unit tests

(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)
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Author: eatoncys <chen.yanshan@zte.com.cn>

Closes #17683 from eatoncys/SPARK-20386.