Add support for matching VLAN fields via rte_flow API.
When matching VLAN pattern, the ethertype field in hardware
filter specification must contain VLAN header's ethertype, and
not Ethernet header's ethertype. The hardware automatically
searches for ethertype 0x8100 in Ethernet header, when
parsing incoming packet against VLAN pattern.
Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>