validation: periodically flush dbcache during reindex-chainstate

Move the periodic flush inside the outer loop of ActivateBestChain.
For very long activations, such as with reindex-chainstate, this
calls periodic flushes so progress can be saved to disk.

Co-Authored-By: l0rinc <pap.lorinc@gmail.com>

src/validation.cpp

@@ -3567,6 +3567,11 @@ bool Chainstate::ActivateBestChain(BlockValidationState& state, std::shared_ptr<
             m_chainman.MaybeRebalanceCaches();
         }
 
+        // Write changes periodically to disk, after relay.
+        if (!FlushStateToDisk(state, FlushStateMode::PERIODIC)) {
+            return false;
+        }
+
         if (WITH_LOCK(::cs_main, return m_disabled)) {
             // Background chainstate has reached the snapshot base block, so exit.
 
@@ -3591,11 +3596,6 @@ bool Chainstate::ActivateBestChain(BlockValidationState& state, std::shared_ptr<
 
     m_chainman.CheckBlockIndex();
 
-    // Write changes periodically to disk, after relay.
-    if (!FlushStateToDisk(state, FlushStateMode::PERIODIC)) {
-        return false;
-    }
-
     return true;
 }