ipv6: fix a possible double free

When calling snmp6_alloc_dev fails, the snmp6 relevant memory
are freed by snmp6_alloc_dev. Calling in6_dev_finish_destroy
will free these memory twice.

Double free will lead that undefined behavior occurs.

Signed-off-by: Roy Li <rongqing.li@windriver.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>