KAFKA-19474: Move WARN log on log truncation below HWM

Moves the check for `offsetTruncationState.offset < log.highWatermark`
before truncating the log as truncation might update the HWM thereby
leading to an incorrect condition evaluation.

Author: gaurav-narula

core/src/main/scala/kafka/server/ReplicaFetcherThread.scala

@@ -166,12 +166,12 @@ class ReplicaFetcherThread(name: String,
     val partition = replicaMgr.getPartitionOrException(tp)
     val log = partition.localLogOrException
 
-    partition.truncateTo(offsetTruncationState.offset, isFuture = false)
-
     if (offsetTruncationState.offset < log.highWatermark)
       warn(s"Truncating $tp to offset ${offsetTruncationState.offset} below high watermark " +
         s"${log.highWatermark}")
 
+    partition.truncateTo(offsetTruncationState.offset, isFuture = false)
+
     // mark the future replica for truncation only when we do last truncation
     if (offsetTruncationState.truncationCompleted)
       replicaMgr.replicaAlterLogDirsManager.markPartitionsForTruncation(brokerConfig.brokerId, tp,