ipv6: addrconf.h cleanups

- Use rcu_dereference_rtnl() in __in6_dev_get
- kerneldoc for __in6_dev_get() and in6_dev_get()
- Use inline functions instead of macros

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>