pktgen: fix xmit test for BQL enabled devices

Similarly as in commit 8e2f1a63 ("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 <dborkman@redhat.com>
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>