KAFKA-15312; Force channel before atomic file move (#14162)

On ext4 file systems we have seen snapshots with zero-length files. This is possible if
the file is closed and moved before forcing the channel to write to disk.

Reviewers: Ron Dagostino <rndgstn@gmail.com>, Alok Thatikunta <athatikunta@confluent.io>

raft/src/main/java/org/apache/kafka/snapshot/FileRawSnapshotWriter.java

@@ -112,6 +112,9 @@ public final class FileRawSnapshotWriter implements RawSnapshotWriter {
             checkIfFrozen("Freeze");
 
             frozenSize = channel.size();
+            // force the channel to write to the file system before closing, to make sure that the file has the data
+            // on disk before performing the atomic file move
+            channel.force(true);
             channel.close();
 
             if (!tempSnapshotPath.toFile().setReadOnly()) {