msg_find_prison
rpr = msg_find_prison(td->td_ucred);
rpr = msg_find_prison(req->td->td_ucred);
rpr = msg_find_prison(td->td_ucred);
rpr = msg_find_prison(td->td_ucred);
if (msg_find_prison(cred) == NULL)
rpr = msg_find_prison(td->td_ucred);
static struct prison *msg_find_prison(struct ucred *);