net: Update the per network namespace sysctls to be available to the network namespace owner

- Allow anyone with CAP_NET_ADMIN rights in the user namespace of the
  the netowrk namespace to change sysctls.
- Allow anyone the uid of the user namespace root the same
  permissions over the network namespace sysctls as the global root.
- Allow anyone with gid of the user namespace root group the same
  permissions over the network namespace sysctl as the global root group.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>