[NETNS][IPV6] proc - protect snmp6 from non-init_net calls

This patchset avoids creation of the /proc entry for snmp6 when
the call is made from a network namespace different from the init_net.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>