Quite often it can be useful to test with dummy or similar
devices as a blackhole sink for skbs. Such devices are only
equipped with a single txq, but marked as NETIF_F_LLTX as
they do not require locking their internal queues on xmit
(or implement locking themselves). Therefore, rather use
HARD_TX_{UN,}LOCK API, so that NETIF_F_LLTX will be respected.

trafgen mmap/TX_RING example against dummy device with config
foo: { fill(0xff, 64) } results in the following performance
improvements for such scenarios on an ordinary Core i7/2.80GHz:

Before:

 Performance counter stats for 'trafgen -i foo -o du0 -n100000000' (10 runs):

   160,975,944,159 instructions:k            #    0.55  insns per cycle          ( +-  0.09% )
   293,319,390,278 cycles:k                  #    0.000 GHz                      ( +-  0.35% )
       192,501,104 branch-misses:k                                               ( +-  1.63% )
               831 context-switches:k                                            ( +-  9.18% )
                 7 cpu-migrations:k                                              ( +-  7.40% )
            69,382 cache-misses:k            #    0.010 % of all cache refs      ( +-  2.18% )
       671,552,021 cache-references:k                                            ( +-  1.29% )

      22.856401569 seconds time elapsed                                          ( +-  0.33% )

After:

 Performance counter stats for 'trafgen -i foo -o du0 -n100000000' (10 runs):

   133,788,739,692 instructions:k            #    0.92  insns per cycle          ( +-  0.06% )
   145,853,213,256 cycles:k                  #    0.000 GHz                      ( +-  0.17% )
        59,867,100 branch-misses:k                                               ( +-  4.72% )
               384 context-switches:k                                            ( +-  3.76% )
                 6 cpu-migrations:k                                              ( +-  6.28% )
            70,304 cache-misses:k            #    0.077 % of all cache refs      ( +-  1.73% )
        90,879,408 cache-references:k                                            ( +-  1.35% )

      11.719372413 seconds time elapsed                                          ( +-  0.24% )

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