[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>
diff --git a/include/net/ipv6.h b/include/net/ipv6.h
index e371f32..3e086f8 100644
--- a/include/net/ipv6.h
+++ b/include/net/ipv6.h
@@ -622,7 +622,7 @@
 extern ctl_table ipv6_route_table[];
 extern ctl_table ipv6_icmp_table[];
 
-extern void ipv6_sysctl_register(void);
+extern int ipv6_sysctl_register(void);
 extern void ipv6_sysctl_unregister(void);
 #endif