Fix race condition for SetBannedSetDirty() calls

Another thread can `SetBannedSetDirty(true)` while `CBanDB::Write()`
call being executed. The following `SetBannedSetDirty(false)`
effectively makes `m_is_dirty` flag value inconsistent with the actual
`m_banned` state. Such behavior can result in data loss, e.g., during
shutdown.

src/banman.cpp

@@ -49,11 +49,12 @@ void BanMan::DumpBanlist()
         SweepBanned();
         if (!BannedSetIsDirty()) return;
         banmap = m_banned;
+        SetBannedSetDirty(false);
     }
 
     int64_t n_start = GetTimeMillis();
-    if (m_ban_db.Write(banmap)) {
-        SetBannedSetDirty(false);
+    if (!m_ban_db.Write(banmap)) {
+        SetBannedSetDirty(true);
     }
 
     LogPrint(BCLog::NET, "Flushed %d banned node addresses/subnets to disk  %dms\n", banmap.size(),