Allocating AF constants for vendors.

[Bruce M. Simpson]

I second Max. If you are going to introduce a bunch of AF_* constants 
into the tree you have to be very careful as AF_MAX is used to size 
arrays and figure out how many radix trie heads to allocate.

It could be argued this wastes a bunch of CPU time and memory, though I 
speculate 'not much' at the moment; I am just a bit concerned that we 
have ifnet->if_afdata which is also sized based on AF_MAX, 37, even 
though most of the protocols in it are never attached to ifnets.

The only domain I've seen which really uses if_afdata is PF_INET6. 
PF_INET does not use it at all. In my opinion, there are structures 
per-family per-ifnet which really belong hung-off ifnet on a 1:1 basis 
and would simplify some of the lazy allocations we have further down in 
the stack.

If AF_MAX increases significantly so will wasted memory. If you are 
going to make any significant changes here, please considering moving 
this stuff to a more dynamic method of allocation.

On the other hand, if you don't need to reference these constants in the 
kernel at all, and they will all exist beyond AF_MAX, then you can 
disregard what I've said and append them to the rest of the list.

That is pretty much what happens for the libpcap/bpf DLT constants 
(which are not an exact analogue of the AF constants - we don't allocate 
other, larger kernel structures based on their value).

regards,
BMS