Fix data race condition in BanMan::DumpBanlist()

The m_is_dirty value being read in BannedSetIsDirty() can differ from
the value being set in SweepBanned(), i.e., be inconsistent with a
BanMan instance internal state.

src/banman.cpp

@@ -43,9 +43,11 @@ void BanMan::DumpBanlist()
     static Mutex dump_mutex;
     LOCK(dump_mutex);
 
-    SweepBanned(); // clean unused entries (if bantime has expired)
-
+    {
+        LOCK(m_cs_banned);
+        SweepBanned();
         if (!BannedSetIsDirty()) return;
+    }
 
     int64_t n_start = GetTimeMillis();