Use mutex pointer instead of name for AssertLockHeld

This makes it useable for non-global locks such as the wallet and
keystore locks.

src/main.cpp

@@ -2238,7 +2238,7 @@ void PushGetBlocks(CNode* pnode, CBlockIndex* pindexBegin, uint256 hashEnd)
 
 bool ProcessBlock(CValidationState &state, CNode* pfrom, CBlock* pblock, CDiskBlockPos *dbp)
 {
-    AssertLockHeld("cs_main");
+    AssertLockHeld(cs_main);
 
     // Check for duplicate
     uint256 hash = pblock->GetHash();