miner: Pass in chainman to RegenerateCommitments

Pass in chainman instead of prev_block so that we can enforce the
block.hashPrevBlock refers to prev_block invariant in the function
itself.

We should probably rethink BlockAssembler's API and somehow include
commitment regeneration functionality in there. Something like a variant
of CreateNewBlock that takes in a std::vector<TxRef> and return a CBlock
instead of CBlockTemplate. That could avoid reaching for
LookupBlockIndex at all.

src/rpc/mining.cpp

@@ -378,8 +378,7 @@ static RPCHelpMan generateblock()
 
     // Add transactions
     block.vtx.insert(block.vtx.end(), txs.begin(), txs.end());
-    CBlockIndex* prev_block = WITH_LOCK(::cs_main, return chainman.m_blockman.LookupBlockIndex(block.hashPrevBlock));
-    RegenerateCommitments(block, prev_block);
+    RegenerateCommitments(block, chainman);
 
     {
         LOCK(cs_main);