ipv6: clean up dev_snmp6 proc entry when we fail to initialize inet6_dev

In ipv6_add_dev, when addrconf_sysctl_register fails, we do not clean up
the dev_snmp6 entry that we have already registered for this device.
Call snmp6_unregister_dev in this case.

Fixes: a317a2f1 ("ipv6: fail early when creating netdev named all or default")
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>