bpf-devices: normalize the return handling of functions that put together policy

bpf-devices: normalize the return handling of functions that put together policy

under some conditions we suppress generating BPF programs. Let's
systematically return 0 when we do this, and 1 if we did actually
soething, instead of second guessing this in the caller.

This is not only more correct, but allows us to suppress BPF programs in
more cases in later commits.