fix(redirect): add IPv6 TPROXY support for TCP redirect listener

Add missing IPV6_TRANSPARENT socket option to the TCP redirect listener
Control callback, matching what the UDP redirect listener already sets.
Without this, the kernel silently drops IPv6 TPROXY-redirected connections.

Also clean up handler_linux.go getOriginalDstAddr:
- Remove encoding/binary import and heap allocation for port conversion
- Replace with zero-allocation ntohs (native-to-network byte order swap)
- Document the struct layout hack where GetsockoptIPv6Mreq and
  GetsockoptIPv6MTUInfo are used to read sockaddr_in/sockaddr_in6
  from SO_ORIGINAL_DST (kernel returns smaller structs than the
  getter functions expect, but only the first N bytes are populated)

Fixes go-gost/gost#126

handler/redirect/tcp/handler_linux.go

@@ -1,7 +1,6 @@
 package redirect
 
 import (
-	"encoding/binary"
 	"errors"
 	"net"
 	"syscall"
@@ -30,6 +29,10 @@ func (h *redirectHandler) getOriginalDstAddr(conn net.Conn) (addr net.Addr, err
 	var cerr error
 	err = rc.Control(func(fd uintptr) {
 		if tcpAddr.IP.To4() != nil {
+			// SO_ORIGINAL_DST returns a struct sockaddr_in (16 bytes).
+			// GetsockoptIPv6Mreq reads into a 20-byte ipv6_mreq; only the
+			// first 16 bytes (the Multiaddr field) are populated.
+			// Layout: [0:2]=sin_family, [2:4]=sin_port (BE), [4:8]=sin_addr.
 			mreq, err := unix.GetsockoptIPv6Mreq(int(fd), unix.IPPROTO_IP, unix.SO_ORIGINAL_DST)
 			if err != nil {
 				cerr = err
@@ -41,17 +44,19 @@ func (h *redirectHandler) getOriginalDstAddr(conn net.Conn) (addr net.Addr, err
 				Port: int(mreq.Multiaddr[2])<<8 + int(mreq.Multiaddr[3]),
 			}
 		} else {
+			// SO_ORIGINAL_DST returns a struct sockaddr_in6 (28 bytes).
+			// GetsockoptIPv6MTUInfo reads into a 32-byte ipv6_mtuinfo; only
+			// the first 28 bytes (the Addr field) are populated.
 			info, err := unix.GetsockoptIPv6MTUInfo(int(fd), unix.IPPROTO_IPV6, unix.SO_ORIGINAL_DST)
 			if err != nil {
 				cerr = err
 				return
 			}
 
-			var buf = make([]byte, 2)
-			binary.BigEndian.PutUint16(buf, info.Addr.Port)
+			// info.Addr.Port is in network byte order (big-endian).
 			addr = &net.TCPAddr{
 				IP:   net.IP(info.Addr.Addr[:]),
-				Port: int(binary.NativeEndian.Uint16(buf)),
+				Port: int(uint16(info.Addr.Port>>8 | info.Addr.Port<<8)),
 			}
 		}
 	})