sysctl: ipv6: use correct net in ipv6_sysctl_rtcache_flush

Before this patch issuing these commands:

  fd = open("/proc/sys/net/ipv6/route/flush")
  unshare(CLONE_NEWNET)
  write(fd, "stuff")

would flush the newly created net, not the original one.

The equivalent ipv4 code is correct (stores the net inside ->extra1).
Acked-by: Daniel Lezcano <daniel.lezcano@free.fr>

Signed-off-by: David S. Miller <davem@davemloft.net>