ipv6: convert the uses of ADBG and remove the superfluous parentheses

Just follow the Joe Perches's opinion, it is a better way to fix the
style errors.

Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Ding Tianhong <dingtianhong@huawei.com>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>