Re: v6: faster tree-based sysctl implementation

I spent some time playing this and managed to get something that works
using proc_dir_entries.  And while it is simpler (600 less lines of
code) it takes about 3x the space of just what using ctl_table entries
does.

I managed to prove to myself that the current sysctl infrastructure
relies the union directory existence semantics pretty strongly.  Despite
all of Al's work to the contrary when he introduced attached_by and kin
in sysctl head.

One nice thing I managed to do was to shift around the problem a bit
so that only at /proc/sys/net do we to namespace weirdness.  Which also
considerably simplifies the problem.

Now that I know that normal unix directory semantics are a lost cause
removing the child entry from ctl_table looks like a very productive
exercise.  

Furthermore it feels like the optimal data structure would be a
directory tree that is created on demand as we create entries,
and a second copy of that directory tree that is per network namespace.

That is very similar to the data structure you wound up with.

So in the next little bit I am going to see if I can combine what
you did and what I did and see if I can come up with something that
is obvious in how it works from looking at it's data structures.

Eric