git: inet6: Take route reference before announcing address

[Roy Marples]

> This is not necessary.  We can remove the same ref++, ref-- sequence
> in rtinit_rtrequest_callback() either.  Back to the time when parallel
> routing was implemented, routing code ran in "route netisr".  But now,
> all routing code runs in netisrs, no need to adjust the ref at more.

Without it I had a panic *I think* in rt_newaddrmsg.
I couldn't get a crashdump and my keyboard became unresponsive - I made 
the assumption it needed to look like rtinit_rtrequest_callback().

After updating sources and commenting out the ref counting in both 
places I can't replicate my panic. I'll commit this shortly.

Roy