[net processing] Inline and simplify UpdatePreferredDownload

We inline `UpdatePreferredDownload` because it is only used in one
location during the version handshake. We simplify it by removing the
initial subtraction of `state->fPreferredDownload` from
`nPreferredDownload`. This is ok since the version handshake is only
called once per peer and `state->fPreferredDownload` will always be
false before the newly inlined code is called, making the subtraction a
noop.

src/net_processing.cpp

@@ -613,8 +613,6 @@ private:
     /** Number of preferable block download peers. */
     int nPreferredDownload GUARDED_BY(cs_main){0};
 
-    void UpdatePreferredDownload(const CNode& node, CNodeState* state) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
-
     bool AlreadyHaveTx(const GenTxid& gtxid) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
     /**
@@ -875,16 +873,6 @@ static void AddKnownTx(Peer& peer, const uint256& hash)
     }
 }
 
-void PeerManagerImpl::UpdatePreferredDownload(const CNode& node, CNodeState* state) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
-{
-    nPreferredDownload -= state->fPreferredDownload;
-
-    // Whether this node should be marked as a preferred download node.
-    state->fPreferredDownload = (!node.IsInboundConn() || node.HasPermission(NetPermissionFlags::NoBan)) && !node.IsAddrFetchConn() && !node.fClient;
-
-    nPreferredDownload += state->fPreferredDownload;
-}
-
 std::chrono::microseconds PeerManagerImpl::NextInvToInbounds(std::chrono::microseconds now,
                                                              std::chrono::seconds average_interval)
 {
@@ -2741,8 +2729,10 @@ void PeerManagerImpl::ProcessMessage(CNode& pfrom, const std::string& msg_type,
 
         // Potentially mark this peer as a preferred download peer.
         {
-        LOCK(cs_main);
-        UpdatePreferredDownload(pfrom, State(pfrom.GetId()));
+            LOCK(cs_main);
+            CNodeState* state = State(pfrom.GetId());
+            state->fPreferredDownload = (!pfrom.IsInboundConn() || pfrom.HasPermission(NetPermissionFlags::NoBan)) && !pfrom.IsAddrFetchConn() && !pfrom.fClient;
+            nPreferredDownload += state->fPreferredDownload;
         }
 
         // Self advertisement & GETADDR logic