RFC: please put back spare fields in struct ifnet (removed in svn 270870)

hi,
sorry if i just noticed it recently.

svn 270870 removed all the if_*spare fields in struct ifnet.
They are replaced with the following comment

/*
 * Spare fields to be added before branching a stable branch, so
 * that structure can be enhanced without changing the kernel
 * binary interface.
 */

​which leaves me a bit unhappy.
Having a stable ABI is useful not only for stable branches,
but also (I would say even more) with head, so people can
run experimental code with limited modifications to the sources.

Cases in point:
- we used one spare field extensively when experimenting
  with netmap, and being able to just build a module without having
  to recompile the whole kernel was a big win.
- we are developing some software GSO and again it was great to have
  the spares in the tcpcb and in the ifnet so we could limit
  modifications to headers used by multiple sources.

I would kindly suggest to put the spares back.
I can't see how they can possibly harm.

thanks
luigi