This function has quite a few parameters. Why not use something like:

static int
wg_aip_addrinfo(struct wg_aip *aip, const void *addr, uint8_t cidr)
{
    switch (aip->af) {
    ...
    }
}

static int
wg_aip_add(...)
{
    ...
    ret = wg_aip_addrinfo(...);
    if (ret != 0) { ... };

    MPASS(af == AF_INET4 || af == AF_INET6);
    root = af == AF_INET ? sc->sc_aip4 : sc->sc_aip6;
    ...
}

Generally because of the next review, where we have to construct an addr/mask before we can find the existing entry.

I had a rough look at the next review as well, and I think it's okay to simplify this function by using an on-stack struct wg_aip aip instead of struct aip_addr a_addr, a_mask.