samples/bpf: xdpsock: Add VLAN support for Tx-only operation

In multi-queue environment testing, the support for VLAN-tag based
steering is useful. So, this patch adds the capability to add
VLAN tag (VLAN ID and Priority) to the generated Tx frame.

To set the VLAN ID=10 and Priority=2 for Tx only through TxQ=3:
 $ xdpsock -i eth0 -t -N -z -q 3 -V -J 10 -K 2

If VLAN ID (-J) and Priority (-K) is set, it default to
  VLAN ID = 1
  VLAN Priority = 0.

For example, VLAN-tagged Tx only, xdp copy mode through TxQ=1:
 $ xdpsock -i eth0 -t -N -c -q 1 -V

Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20211230035447.523177-2-boon.leong.ong@intel.com