conn: store IP_PKTINFO cmsg in StdNetendpoint src

Replace the src storage inside StdNetEndpoint with a copy of the raw
control message buffer, to reduce allocation and perform less work on a
per-packet basis.

Signed-off-by: James Tucker <james@tailscale.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

conn/sticky_default.go

@@ -7,6 +7,20 @@
 
 package conn
 
+import "net/netip"
+
+func (e *StdNetEndpoint) SrcIP() netip.Addr {
+	return netip.Addr{}
+}
+
+func (e *StdNetEndpoint) SrcIfidx() int32 {
+	return 0
+}
+
+func (e *StdNetEndpoint) SrcToString() string {
+	return ""
+}
+
 // TODO: macOS, FreeBSD and other BSDs likely do support this feature set, but
 // use alternatively named flags and need ports and require testing.