np->ucast_oif is read locklessly in some contexts.

Make all accesses to this field lockless, adding appropriate
annotations.

This also makes setsockopt( IPV6_UNICAST_IF ) lockless.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>