ipv6: icmp: use icmpv6_sk_exit()

Simply use icmpv6_sk_exit() when inet_ctl_sock_create() fail
in icmpv6_sk_init().

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>