This patch restores cake's deployed behavior at line rate to always
split gso, and makes gso splitting configurable from userspace.

running cake unlimited (unshaped) at 1gigE, local traffic:

no-split-gso bql limit: 131966
split-gso bql limit:   ~42392-45420

On this 4 stream test splitting gso apart results in halving the
observed interpacket latency at no loss in throughput.

Summary of tcp_nup test run 'gso-split' (at 2018-07-26 16:03:51.824728):

 Ping (ms) ICMP :         0.83         0.81 ms              341
 TCP upload avg :       235.43       235.39 Mbits/s         301
 TCP upload sum :       941.71       941.56 Mbits/s         301
 TCP upload::1  :       235.45       235.43 Mbits/s         271
 TCP upload::2  :       235.45       235.41 Mbits/s         289
 TCP upload::3  :       235.40       235.40 Mbits/s         288
 TCP upload::4  :       235.41       235.40 Mbits/s         291

verses

Summary of tcp_nup test run 'no-split-gso' (at 2018-07-26 16:37:23.563960):

                           avg       median          # data pts
 Ping (ms) ICMP :         1.67         1.73 ms              348
 TCP upload avg :       234.56       235.37 Mbits/s         301
 TCP upload sum :       938.24       941.49 Mbits/s         301
 TCP upload::1  :       234.55       235.38 Mbits/s         285
 TCP upload::2  :       234.57       235.37 Mbits/s         286
 TCP upload::3  :       234.58       235.37 Mbits/s         274
 TCP upload::4  :       234.54       235.42 Mbits/s         288
Signed-off-by: David S. Miller <davem@davemloft.net>