[net] Make cs_inventory a non-recursive mutex

cs_inventory is never taken recursively. Make it a non-recursive mutex.

src/net.h

@@ -803,7 +803,7 @@ public:
     // There is no final sorting before sending, as they are always sent immediately
     // and in the order requested.
     std::vector<uint256> vInventoryBlockToSend GUARDED_BY(cs_inventory);
-    RecursiveMutex cs_inventory;
+    Mutex cs_inventory;
 
     struct TxRelay {
         mutable RecursiveMutex cs_filter;