ipv6: remove a rcu_read_lock in ndisc_constructor

in6_dev_get(dev) takes a reference on struct inet6_dev, we dont need
rcu locking in ndisc_constructor()

Signed-off-by: Roy.Li <rongqing.li@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>