3747 txg commit callbacks don't work
Submitted by:   Will Andrews <willa@spectralogic.com>
Reviewed by:    Matthew Ahrens <mahrens@delphix.com>

@@ -4511,11 +4511,11 @@
          * commit callback hasn't been called, then most likely there's an
          * implementation bug..
          */
         tmp_cb = list_head(&zcl.zcl_callbacks);
         if (tmp_cb != NULL &&
-            tmp_cb->zcd_txg > txg - ZTEST_COMMIT_CALLBACK_THRESH) {
+            (txg - ZTEST_COMMIT_CALLBACK_THRESH) > tmp_cb->zcd_txg) {
                 fatal(0, "Commit callback threshold exceeded, oldest txg: %"
                     PRIu64 ", open txg: %" PRIu64 "\n", tmp_cb->zcd_txg, txg);
         }
 
         /*