pf: Improve ioctl() input validation

[kp]

Summary

Both DIOCCHANGEADDR and DIOCADDADDR take a struct pf_pooladdr from
userspace. They failed to validate the dyn pointer contained in its
struct pf_addr_wrap member structure.

This triggered assertion failures under fuzz testing in
pfi_dynaddr_setup(). Happily the dyn variable was overruled there, but
we should verify that it's set to NULL anyway.

Reported-by: [e-mail]

[emaste]

Seems fine to me, although I'm wondering about having the kernel set it to NULL on entry vs requiring that it's NULL.

[kp]

I think we want to check for NULL, as a robustness / future proofing measure.
Imagine that we eventually want to start using this variable for input (I don't know why, but for the sake of argument...), if we accepted random input there we could never tell the difference between an old binary (pre-using 'dyn' as input) passing garbage, and a new one passing a valid value.