fix: return buffers to pool on error paths to reduce GC pressure (#5203)

- pkg/nathole/nathole.go: add pool.PutBuf(buf) on ReadFromUDP error
  and DecodeMessageInto error paths in waitDetectMessage
- pkg/proto/udp/udp.go: add defer pool.PutBuf(buf) in writerFn to
  ensure buffer is returned when the goroutine exits

pkg/nathole/nathole.go

@@ -298,11 +298,13 @@ func waitDetectMessage(
 		n, raddr, err := conn.ReadFromUDP(buf)
 		_ = conn.SetReadDeadline(time.Time{})
 		if err != nil {
+			pool.PutBuf(buf)
 			return nil, err
 		}
 		xl.Debugf("get udp message local %s, from %s", conn.LocalAddr(), raddr)
 		var m msg.NatHoleSid
 		if err := DecodeMessageInto(buf[:n], key, &m); err != nil {
+			pool.PutBuf(buf)
 			xl.Warnf("decode sid message error: %v", err)
 			continue
 		}

pkg/proto/udp/udp.go

@@ -85,6 +85,7 @@ func Forwarder(dstAddr *net.UDPAddr, readCh <-chan *msg.UDPPacket, sendCh chan<-
 		}()
 
 		buf := pool.GetBuf(bufSize)
+		defer pool.PutBuf(buf)
 		for {
 			_ = udpConn.SetReadDeadline(time.Now().Add(30 * time.Second))
 			n, _, err := udpConn.ReadFromUDP(buf)