ORC-1530: Rename variables in RecordReaderUtils.ChunkReader#create

### What changes were proposed in this pull request?

rename variables in RecordReaderUtils.ChunkReader#create

### Why are the changes needed?

for easy reading

see [PR 1662](#1662 (comment))

### How was this patch tested?

Closes #1664 from yebukong/ORC-1528_rename_variables.

Authored-by: yebukong <yebukong@qq.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

java/core/src/java/org/apache/orc/impl/RecordReaderUtils.java

@@ -770,32 +770,32 @@ void readRanges(FSDataInputStream file, boolean allocateDirect, double extraByte
     }
 
     static ChunkReader create(BufferChunk from, BufferChunk to) {
-      long f = Long.MAX_VALUE;
-      long e = Long.MIN_VALUE;
+      long start = Long.MAX_VALUE;
+      long end = Long.MIN_VALUE;
 
-      long cf = Long.MAX_VALUE;
-      long ef = Long.MIN_VALUE;
+      long currentStart = Long.MAX_VALUE;
+      long currentEnd = Long.MIN_VALUE;
       long reqBytes = 0L;
 
       BufferChunk current = from;
       while (current != to.next) {
-        f = Math.min(f, current.getOffset());
-        e = Math.max(e, current.getEnd());
-        if (ef == Long.MIN_VALUE || current.getOffset() <= ef) {
-          cf = Math.min(cf, current.getOffset());
-          ef = Math.max(ef, current.getEnd());
+        start = Math.min(start, current.getOffset());
+        end = Math.max(end, current.getEnd());
+        if (currentEnd == Long.MIN_VALUE || current.getOffset() <= currentEnd) {
+          currentStart = Math.min(currentStart, current.getOffset());
+          currentEnd = Math.max(currentEnd, current.getEnd());
         } else {
-          reqBytes += ef - cf;
-          cf = current.getOffset();
-          ef = current.getEnd();
+          reqBytes += currentEnd - currentStart;
+          currentStart = current.getOffset();
+          currentEnd = current.getEnd();
         }
         current = (BufferChunk) current.next;
       }
-      reqBytes += ef - cf;
+      reqBytes += currentEnd - currentStart;
       if (reqBytes > IOUtils.MAX_ARRAY_SIZE) {
         throw new IllegalArgumentException("invalid reqBytes value " + reqBytes + ",out of bounds " + IOUtils.MAX_ARRAY_SIZE);
       }
-      long readBytes = e - f;
+      long readBytes = end - start;
       if (readBytes > IOUtils.MAX_ARRAY_SIZE) {
         throw new IllegalArgumentException("invalid readBytes value " + readBytes + ",out of bounds " + IOUtils.MAX_ARRAY_SIZE);
       }