[SPARK-10399][SPARK-23879][FOLLOWUP][CORE] Free unused off-heap memor…

…y in MemoryBlockSuite

## What changes were proposed in this pull request?

As viirya pointed out [here](#19222 (comment)), this PR explicitly frees unused off-heap memory in `MemoryBlockSuite`

## How was this patch tested?

Existing UTs

Author: Kazuaki Ishizaki <ishizaki@jp.ibm.com>

Closes #21117 from kiszk/SPARK-10399-free-offheap.

common/unsafe/src/test/java/org/apache/spark/unsafe/memory/MemoryBlockSuite.java

@@ -120,6 +120,8 @@ private void check(MemoryBlock memory, Object obj, long offset, int length) {
     } catch (Exception expected) {
       Assert.assertThat(expected.getMessage(), containsString("should not be larger than"));
     }
+
+    memory.setPageNumber(MemoryBlock.NO_PAGE_NUMBER);
   }
 
   @Test
@@ -165,11 +167,13 @@ public void testOffHeapArrayMemoryBlock() {
     int length = 56;
 
     check(memory, obj, offset, length);
+    memoryAllocator.free(memory);
 
     long address = Platform.allocateMemory(112);
     memory = new OffHeapMemoryBlock(address, length);
     obj = memory.getBaseObject();
     offset = memory.getBaseOffset();
     check(memory, obj, offset, length);
+    Platform.freeMemory(address);
   }
 }