Call onDispose before first read

The cancellation callback in asynchronousReadFileChannel must be called
before the first read I/O or otherwise if cancellation signals happens
immediately the onDispose callback may be missed.

The DefaultBufferFactory workaround however remains in place until an
expected additional fix arrives with Reactor Core 3.2.9.

See gh-22107

Author: rstoyanchev

spring-core/src/main/java/org/springframework/core/io/buffer/DataBufferUtils.java

@@ -144,10 +144,10 @@ public static Flux<DataBuffer> readAsynchronousFileChannel(Callable<Asynchronous
 				channel -> Flux.create(sink -> {
 					ReadCompletionHandler handler =
 							new ReadCompletionHandler(channel, sink, position, bufferFactoryToUse, bufferSize);
+					sink.onDispose(handler::dispose);
 					DataBuffer dataBuffer = bufferFactoryToUse.allocateBuffer(bufferSize);
 					ByteBuffer byteBuffer = dataBuffer.asByteBuffer(0, bufferSize);
 					channel.read(byteBuffer, position, dataBuffer, handler);
-					sink.onDispose(handler::dispose);
 				}),
 				channel -> {
 					// Do not close channel from here, rather wait for the current read callback