[SPARK-21090][CORE] Optimize the unified memory manager code

## What changes were proposed in this pull request?
1.In `acquireStorageMemory`, when the Memory Mode is OFF_HEAP ,the `maxOffHeapMemory` should be modified to `maxOffHeapStorageMemory`. after this PR,it will same as ON_HEAP Memory Mode.
Because when acquire memory is between `maxOffHeapStorageMemory` and `maxOffHeapMemory`,it will fail surely, so if acquire memory is greater than  `maxOffHeapStorageMemory`(not greater than `maxOffHeapMemory`),we should fail fast.
2. Borrow memory from execution, `numBytes` modified to `numBytes - storagePool.memoryFree` will be more reasonable.
Because we just acquire `(numBytes - storagePool.memoryFree)`, unnecessary borrowed `numBytes` from execution

## How was this patch tested?
added unit test case

Author: liuxian <liu.xian3@zte.com.cn>

Closes #18296 from 10110346/wip-lx-0614.