save to call if_attachdomain from if_attach() (as done for if_loop.c). We
will now end up with a properly initialized if_afdata array and the nd6
callout will no longer try to deref a NULL pointer.
Still this is a temp workaround and the locking for if_afdata should be
revisited at a later point.
Requested by: rwatson
Discussed with and tested by: yongari (a while ago)
PR: kern/70393
MFC after: 5 days