We link `ifp' to `if_list' before we perform if_attachsetup(). It is not

We link `ifp' to `if_list' before we perform if_attachsetup(). It is not
fully initialized because we initialize `if_groups' after linking. It's
not triggered because if_attach() and if_unit(9) are serialized by
kernel lock and `ifp' is often filled by nulls. Move `if_groups'
initialization to if_attach_common() to prevent this.

ok bluhm@ claudio@ deraadt@