Expecting backdoors is a bit strong...
There are several strong arguments against the plausibility of such backdoors:
Linux is used by a lot of people, including US corporations. A big part of the mandate of modern security agencies is to protect the interests of their country. In particular, the NSA shall, as much as possible, protect US corporations against spying from foreign competitors. Putting a backdoor in Linux implies the risk of allowing "bad people" (from the NSA point of view) to spy on US corporations through this backdoor.
Linux is open-source and the kernel is believed to be under rather thorough scrutiny from competent programmers. This is the "many eyes" theory. SELinux is right in the middle of all this inspection. Whether the "many eyes" theory actually holds is debatable (and debated). However, there are people who do PhD theses on SELinux, so it is not preposterous to assume that this particular piece of code was thoroughly investigated.
Any patch committed into the Linux kernel is followed through revision control. SELinux comes from the NSA and is tagged as such. If a backdoor was inserted and then subsequently discovered, it would be easy to track it back to the apparent author. A very basic protection measure is to not do such things in your own name ! If I were the NSA, I would first build up a virtual persona who is not associated with the NSA, so that even if he gets caught pushing backdoors, this will not incriminate my organization. Spy agencies know a lot about spy network segmentation. It would be singularly dumb of them to inject backdoors in their own name.
There is also a strong argument for the existence of such backdoors:
- Spying on a lot of people and organizations is the core business of the NSA.
Honestly, until you find the corpse (i.e. the backdoor itself), your question is unanswerable. It is a matter of many parameters which can only be know through subjective estimates...
(Personally, I still find that backdoors in PRNG, especially hardware PRNG, are much more plausible than backdoors "hidden in plain sight".)