pktgen: be friendly to LLTX devices

Similarly to commit 43279500

 ("packet: respect devices with
LLTX flag in direct xmit"), we can basically apply the very same
to pktgen. This will help testing against LLTX devices such as
dummy driver (or others), which only have a single netdevice txq
and would otherwise require locking their txq from pktgen side
while e.g. in dummy case, we would not need any locking. Fix this
by making use of HARD_TX_{UN,}LOCK API, so that NETIF_F_LLTX will
be respected.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>