Require WriteUndoDataForBlock() to hold mutex cs_main

Summary:
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().

This is a partial backport of [[bitcoin/bitcoin#22932 | core#22932]]
bitcoin/bitcoin@2e557ce

Depends on D13031

Test Plan:
With clang and DEBUG:

`ninja all check-all`

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13032

src/node/blockstorage.cpp

@@ -698,6 +698,7 @@ 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;

src/node/blockstorage.h

@@ -11,8 +11,11 @@
 #include <chain.h>
 #include <fs.h>
 #include <protocol.h> // For CMessageHeader::MessageStartChars
+#include <sync.h>
 #include <txdb.h>
 
+extern RecursiveMutex cs_main;
+
 class ArgsManager;
 class BlockValidationState;
 class CBlock;
@@ -173,7 +176,8 @@ class BlockManager {
 
     bool WriteUndoDataForBlock(const CBlockUndo &blockundo,
                                BlockValidationState &state, CBlockIndex *pindex,
-                               const CChainParams &chainparams);
+                               const CChainParams &chainparams)
+        EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
     FlatFilePos SaveBlockToDisk(const CBlock &block, int nHeight,
                                 CChain &active_chain,