pktgen: Allow on loopback device

When pktgen is used to measure the performance of dev_queue_xmit()
packet handling in the core, it is preferable to not hand down
packets to a low-level Ethernet driver as it would distort the
measurements.

Allow using pktgen on the loopback device, thus constraining
measurements to core code.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: David S. Miller <davem@davemloft.net>