jail: Use atomic(9) instead of CK atomics

[markj]

Summary

There's no reason to use one over the other here, let's prefer the
interface that's used elsewhere in the kernel.

No functional change intended.

[glebius]

My motivation to use these primitives was his: "The network epoch protects some ck lists, now it is going to also protect some pointers, so let's use ck primitive for that.". I don't have any strong opinion on this. So I neither approve nor block this change. No objection from me.