Correct index path validation (elastic#129144)

All we care about is if reindex is true or false. We shouldn't worry
about force merge. Because if reindex is true, we will create the
directory, if its false, we won't.

Author: benwtrent

qa/vector/src/main/java/org/elasticsearch/test/knn/KnnIndexTester.java

@@ -177,13 +177,8 @@ public static void main(String[] args) throws Exception {
                     cmdLineArgs.vectorSpace(),
                     cmdLineArgs.numDocs()
                 );
-                if (Files.exists(indexPath) == false) {
-                    if (cmdLineArgs.reindex() == false) {
-                        throw new IllegalArgumentException("Index path does not exist: " + indexPath);
-                    }
-                    if (cmdLineArgs.forceMerge()) {
-                        throw new IllegalArgumentException("Force merging without an existing index in: " + indexPath);
-                    }
+                if (cmdLineArgs.reindex() == false && Files.exists(indexPath) == false) {
+                    throw new IllegalArgumentException("Index path does not exist: " + indexPath);
                 }
                 if (cmdLineArgs.reindex()) {
                     knnIndexer.createIndex(result);

qa/vector/src/main/java/org/elasticsearch/test/knn/KnnIndexer.java

@@ -304,7 +304,7 @@ private void readNext() throws IOException {
                 bytes.position(0);
                 // wrap around back to the start of the file if we hit the end:
                 logger.warn("VectorReader hit EOF when reading " + this.input + "; now wrapping around to start of file again");
-                this.input.position(position);
+                input.position(position);
                 bytesRead = Channels.readFromFileChannel(this.input, position, bytes);
                 if (bytesRead < bytes.capacity()) {
                     throw new IllegalStateException(