Google should strip out unreferenced symbols... but it probably doesn't matter

The primary motivator in subsetting the kernel headers is to end up with a minimal set of headers necessary to encompass the userspace<->kernel interface surface, and avoid clutter, confusion, or future brokenness by avoiding exposing userspace to anything more than is strictly necessary to make calls into the kernel.

From a support standpoint, anything in headers that doesn't need to be there (as far as structures, function declarations, etc) is something that might break when we refresh the headers from a newer kernel version.

Knowing the habits of application developers, if we didn't yank the #if __KERNEL__ sections, *somebody* would inevitably write code that #define __KERNEL__ 1 and included some linux header for some absurd reason and then they'd be cranky if it stopped working in the future.