ipvlan: properly annotate rx_handler access

The rx_handler field is rcu-protected, but I forgot to use the
proper accessor while refactoring netif_is_ipvlan_port(). Such
function only check the rx_handler value, so it is safe, but we need
to properly read rx_handler via rcu_access_pointer() to avoid sparse
warnings.

Fixes: 1ec54cb4 ("net: unpollute priv_flags space")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>