net: Updates to netif_index_is_vrf

As Eric noted netif_index_is_vrf is not called with rcu_read_lock held,
so wrap the dev_get_by_index_rcu in rcu_read_lock and unlock.

If VRF is not enabled or oif is 0 skip the device lookup. In both cases
index cannot be the VRF master.

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>