sh_eth: RX checksum offload support

Add support for the RX checksum offload. This is enabled by default and
may be disabled and re-enabled using 'ethtool':

# ethtool -K eth0 rx off
# ethtool -K eth0 rx on

Some Ether MACs provide a simple checksumming scheme which appears to be
completely compatible with CHECKSUM_COMPLETE: sum of all packet data after
the L2 header is appended to packet data; this may be trivially read by
the driver and used to update the skb accordingly. The same checksumming
scheme is implemented in the EtherAVB MACs and now supported by the 'ravb'
driver.

In terms of performance, throughput is close to gigabit line rate with the
RX checksum offload both enabled and disabled.  The 'perf' output, however,
appears to indicate that significantly less time is spent in do_csum() --
this is as expected.

Test results with RX checksum offload enabled:

~/netperf-2.2pl4# perf record -a ./netperf -t TCP_MAERTS -H 192.168.2.4
TCP MAERTS TEST to 192.168.2.4
Recv   Send    Send
Socket Socket  Message  Elapsed
Size   Size    Size     Time     Throughput
bytes  bytes   bytes    secs.    10^6bits/sec

131072  16384  16384    10.01     933.93
[ perf record: Woken up 8 times to write data ]
[ perf record: Captured and wrote 1.955 MB perf.data (41940 samples) ]
~/netperf-2.2pl4# perf report
Samples: 41K of event 'cycles:ppp', Event count (approx.): 9915302763
Overhead  Command          Shared Object             Symbol
   9.44%  netperf          [kernel.kallsyms]         [k] __arch_copy_to_user
   7.75%  swapper          [kernel.kallsyms]         [k] _raw_spin_unlock_irq
   6.31%  swapper          [kernel.kallsyms]         [k] default_idle_call
   5.89%  swapper          [kernel.kallsyms]         [k] arch_cpu_idle
   4.37%  swapper          [kernel.kallsyms]         [k] tick_nohz_idle_exit
   4.02%  netperf          [kernel.kallsyms]         [k] _raw_spin_unlock_irq
   2.52%  netperf          [kernel.kallsyms]         [k] preempt_count_sub
   1.81%  netperf          [kernel.kallsyms]         [k] tcp_recvmsg
   1.80%  netperf          [kernel.kallsyms]         [k] _raw_spin_unlock_irqres
   1.78%  netperf          [kernel.kallsyms]         [k] preempt_count_add
   1.36%  netperf          [kernel.kallsyms]         [k] __tcp_transmit_skb
   1.20%  netperf          [kernel.kallsyms]         [k] __local_bh_enable_ip
   1.10%  netperf          [kernel.kallsyms]         [k] sh_eth_start_xmit

Test results with RX checksum offload disabled:

~/netperf-2.2pl4# perf record -a ./netperf -t TCP_MAERTS -H 192.168.2.4
TCP MAERTS TEST to 192.168.2.4
Recv   Send    Send
Socket Socket  Message  Elapsed
Size   Size    Size     Time     Throughput
bytes  bytes   bytes    secs.    10^6bits/sec
131072  16384  16384    10.01     932.04
[ perf record: Woken up 14 times to write data ]
[ perf record: Captured and wrote 3.642 MB perf.data (78817 samples) ]
~/netperf-2.2pl4# perf report
Samples: 78K of event 'cycles:ppp', Event count (approx.): 18091442796
Overhead  Command          Shared Object       Symbol
   7.00%  swapper          [kernel.kallsyms]   [k] do_csum
   3.94%  swapper          [kernel.kallsyms]   [k] sh_eth_poll
   3.83%  ksoftirqd/0      [kernel.kallsyms]   [k] do_csum
   3.23%  swapper          [kernel.kallsyms]   [k] _raw_spin_unlock_irq
   2.87%  netperf          [kernel.kallsyms]   [k] __arch_copy_to_user
   2.86%  swapper          [kernel.kallsyms]   [k] arch_cpu_idle
   2.13%  swapper          [kernel.kallsyms]   [k] default_idle_call
   2.12%  ksoftirqd/0      [kernel.kallsyms]   [k] sh_eth_poll
   2.02%  swapper          [kernel.kallsyms]   [k] _raw_spin_unlock_irqrestore
   1.84%  swapper          [kernel.kallsyms]   [k] __softirqentry_text_start
   1.64%  swapper          [kernel.kallsyms]   [k] tick_nohz_idle_exit
   1.53%  netperf          [kernel.kallsyms]   [k] _raw_spin_unlock_irq
   1.32%  netperf          [kernel.kallsyms]   [k] preempt_count_sub
   1.27%  swapper          [kernel.kallsyms]   [k] __pi___inval_dcache_area
   1.22%  swapper          [kernel.kallsyms]   [k] check_preemption_disabled
   1.01%  ksoftirqd/0      [kernel.kallsyms]   [k] _raw_spin_unlock_irqrestore

The above results collected on the R-Car V3H Starter Kit board.

Based on the commit 4d86d381

 ("ravb: RX checksum offload")...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>