I think it's worth noting that a pretty big percentage of the if's in the kernel are one line. I'm not particularly tied to one opinion or the other, I'll do whatever fits with the project (Though I do tend to use the one line if syntax for personal projects). But, I personally like them in the kernel's source simply because one line if's are so common and mostly encouraged. IMO, a better solution is to use a context-aware patch system, rather then line-based patches. That brings it's own set of problems though unfortunately.

That said, I think the argument does apply in that some pieces of the kernel don't strictly follow the kernel style, and the fact that braces aren't enforced leads to some uglier pieces of code [0] being allowed despite not strictly adhering to the style.

[0] https://github.com/torvalds/linux/blob/master/kernel/groups....