Keep nodes above watermark in testAutomaticReleaseOfIndexBlock (#47387)

Today the comment boldly claims that this line of code keeps nodes above the
10-byte low watermark when in fact this is not true at all. This change fixes
this so that it really does keep nodes above the low watermark.

Fixes #45338. Again.

server/src/test/java/org/elasticsearch/cluster/routing/allocation/decider/MockDiskUsagesIT.java

@@ -185,7 +185,7 @@ public void testAutomaticReleaseOfIndexBlock() throws Exception {
  // Move all nodes above the low watermark so no shard movement can occur, and at least one node above the flood stage watermark so
  // the index is blocked
  clusterInfoService.diskUsageFunction = (discoveryNode, fsInfoPath) -> setDiskUsage(fsInfoPath, 100,
- discoveryNode.getId().equals(nodeIds.get(2)) ? between(0, 4) : between(0, 14));
+ discoveryNode.getId().equals(nodeIds.get(2)) ? between(0, 4) : between(0, 9));
 
  assertBusy(() -> assertBlocked(
  client().prepareIndex().setIndex("test").setType("doc").setId("1").setSource("foo", "bar"),