net: handle 802.1P vlan 0 packets properly

When stack receives pkt: [802.1P vlan 0][802.1AD vlan 100][IPv4],
vlan_do_receive() returns false if it does not find vlan_dev. Later
__netif_receive_skb_core() fails to find packet type handler for
skb->protocol 801.1AD and drops the packet.

801.1P header with vlan id 0 should be handled as untagged packets.
This patch fixes it by checking if vlan_id is 0 and processes next vlan
header.

Signed-off-by: Govindarajulu Varadarajan <gvaradar@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>