[PKTGEN]: vlan support

The attached patch allows pktgen to produce 802.1Q and Q-in-Q tagged frames.
I have used it for stress test a bridge and seems ok to me.
Unfortunately I have no access to net-2.6.x git tree so the diff is against
2.6.17.13.

Signed-off-by: Francesco Fondelli <francesco.fondelli@gmail.com>
Acked-by:      Steven Whitehouse <steve@chygwyn.com>
Signed-off-by: Robert Olsson <robert.olsson@its.uu.se>
Signed-off-by: David S. Miller <davem@davemloft.net>