IPv4, IPv6 and link-layer multicast refcounting in bms_netdev

[Bruce M Simpson]

I have just committed reference counting for multicast structures in p4. 
Change list number is 116036.

This should fix the problems with pfsync and carp since the scalability 
fixes for IPv4 multicast last September. A further cumulative fix for 
pfsync is present in this branch.

Basic testing with the stock IPv4 and Ethernet code have been performed. 
Further testing would be much appreciated before the code is merged to 
HEAD. The refcounting has been implemented in a way so as not to break 
the 6.x ABI so that it may be merged to STABLE.

It would be great to have feedback on how these patches may affect 
vlan(4) which is the only other consumer of the in_delmulti() KPI.

My experience working on this suggests IFF_NEEDSGIANT is a real headache 
for dealing with ifnets which may potentially go away during the 
lifetime of the system.

Regards,
BMS