[NETNS][IPV6]: Make ipv6_sysctl_register to return a value.

This patch makes the function ipv6_sysctl_register to return a
value. The af_inet6 init function is now able to handle an error and
catch it from the initialization of the sysctl.

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