Fix race condition for AsynchronousFileChannel

This commit fixes an issue in the DataBufferUtils.write variant that
takes a AsynchronousFileChannel.

Issue: SPR-15798

spring-core/src/test/java/org/springframework/core/io/buffer/DataBufferUtilsTests.java

@@ -207,7 +207,6 @@ public class DataBufferUtilsTests extends AbstractDataBufferAllocatingTestCase {
 	}
 
 	@Test
-	@Ignore  // SPR-15798
 	public void writeAsynchronousFileChannel() throws Exception {
 		DataBuffer foo = stringBuffer("foo");
 		DataBuffer bar = stringBuffer("bar");