kvs: call content.flush before checkpoint

Problem: When the KVS module is unloaded, a checkpoint of the root
reference is attempted.  However, a content.flush is not done
beforehand.  This could result in an invalid checkpoint reference
as data is not guaranteed to be flushed to the backing store.

Solution: Call content.flush before checkpointing.

Fixes #6237

src/modules/kvs/kvs.c

@@ -3142,15 +3142,22 @@ static int checkpoint_get (flux_t *h, char *buf, size_t len, int *seq)
  */
 static int checkpoint_put (flux_t *h, const char *rootref, int rootseq)
 {
-    flux_future_t *f = NULL;
+    flux_future_t *f1 = NULL;
+    flux_future_t *f2 = NULL;
     int rv = -1;
 
-    if (!(f = kvs_checkpoint_commit (h, NULL, rootref, rootseq, 0, 0))
-        || flux_rpc_get (f, NULL) < 0)
+    /* first must ensure all content is flushed */
+    if (!(f1 = flux_rpc (h, "content.flush", NULL, 0, 0))
+        || flux_rpc_get (f1, NULL) < 0)
+        goto error;
+
+    if (!(f2 = kvs_checkpoint_commit (h, NULL, rootref, rootseq, 0, 0))
+        || flux_rpc_get (f2, NULL) < 0)
         goto error;
     rv = 0;
 error:
-    flux_future_destroy (f);
+    flux_future_destroy (f1);
+    flux_future_destroy (f2);
     return rv;
 }