net: missing bits of net-namespace / sysctl

Piss-poor sysctl registration API strikes again, film at 11...
What we really need is _pathname_ required to be present in
already registered table, so that kernel could warn about bad
order.  That's the next target for sysctl stuff (and generally
saner and more explicit order of initialization of ipv[46]
internals wouldn't hurt either).

For the time being, here are full fixups required by ..._rotable()
stuff; we make per-net sysctl sets descendents of "ro" one and
make sure that sufficient skeleton is there before we start registering
per-net sysctls.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>