mpd5/Netgraph issues after upgrading to 7.4

[Przemyslaw Frasunek]

> I suspect this isn't related to netgraph, but to IPv6 since prelist_remove()
> is found in netinet6/nd6_rtr.c.
> 
> Several times I looked into ND code and found lots of race prone code there.
> May be some was recently fixed by bz@, but definitely not merged to stable/8.

Thanks a lot guys. For now, I disabled IPv6 on this BRAS. Let's see if it's
going to help.