pktgen: fix xmit test for BQL enabled devices

pktgen: fix xmit test for BQL enabled devices

Similarly as in commit 8e2f1a63f221 ("packet: fix packet_direct_xmit
for BQL enabled drivers"), we test for __QUEUE_STATE_STACK_XOFF bit
in pktgen's xmit, which would not fully fill the device's TX ring for
BQL drivers that use netdev_tx_sent_queue(). Fix is to use, similarly
as we do in packet sockets, netif_xmit_frozen_or_drv_stopped() test.

Signed-off-by: Daniel Borkmann <[email protected]>
Cc: Eric Dumazet <[email protected]>
Signed-off-by: David S. Miller <[email protected]>