When you think about the utility of such backdoors, the ones in Windows like in Bitlocker are much more useful to intelligence agencies than a similar thing in Linux. As Linux is mainly installed on a server, we are talking about a backdoor that allows defined or arbitrary code execution based on a given signal such as specific connection data. What happens next is also important (a connection is established, a hidden process is created and so on). When you take all this into consideration, hiding a backdoor in the Linux kernel is not the most efficient way to proceed, especially when you normally deal with specific targets. So instead of a full backdoor you would prefer introducing a feature that would play nicely with a zero day you already developed.