[FLINK-22385][runtime] Fixes type cast error

This closes #15710.

Author: sharkdtu

flink-runtime/src/main/java/org/apache/flink/runtime/io/network/buffer/NetworkBufferPool.java

@@ -285,7 +285,7 @@ public int getTotalNumberOfMemorySegments() {
     }
 
     public long getTotalMemory() {
-        return getTotalNumberOfMemorySegments() * memorySegmentSize;
+        return (long) getTotalNumberOfMemorySegments() * memorySegmentSize;
     }
 
     public int getNumberOfAvailableMemorySegments() {
@@ -295,15 +295,15 @@ public int getNumberOfAvailableMemorySegments() {
     }
 
     public long getAvailableMemory() {
-        return getNumberOfAvailableMemorySegments() * memorySegmentSize;
+        return (long) getNumberOfAvailableMemorySegments() * memorySegmentSize;
     }
 
     public int getNumberOfUsedMemorySegments() {
         return getTotalNumberOfMemorySegments() - getNumberOfAvailableMemorySegments();
     }
 
     public long getUsedMemory() {
-        return getNumberOfUsedMemorySegments() * memorySegmentSize;
+        return (long) getNumberOfUsedMemorySegments() * memorySegmentSize;
     }
 
     public int getNumberOfRegisteredBufferPools() {