Require WriteUndoDataForBlock() to hold mutex cs_main

Mutex cs_main is already held by the caller of WriteUndoDataForBlock().
This change is needed to require CBlockIndex::GetUndoPos() to hold
cs_main and CBlockIndex::nStatus to be guarded by cs_main in the
following commits without adding 2 unnecessary cs_main locks to
WriteUndoDataForBlock().

src/node/blockstorage.cpp

@@ -712,6 +712,7 @@ static bool WriteBlockToDisk(const CBlock& block, FlatFilePos& pos, const CMessa
 
 bool BlockManager::WriteUndoDataForBlock(const CBlockUndo& blockundo, BlockValidationState& state, CBlockIndex* pindex, const CChainParams& chainparams)
 {
+    AssertLockHeld(::cs_main);
     // Write undo information to disk
     if (pindex->GetUndoPos().IsNull()) {
         FlatFilePos _pos;