This patch makes use of build_skb() throughout in driver's receieve
data path [HW gro flow and non HW gro flow]. With this, driver can
build skb directly from the page segments which are already mapped
to the hardware instead of allocating new SKB via netdev_alloc_skb()
and memcpy the data which is quite costly.

This really improves performance (keeping same or slight gain in rx
throughput) in terms of CPU utilization which is significantly reduced
[almost half] in non HW gro flow where for every incoming MTU sized
packet driver had to allocate skb, memcpy headers etc. Additionally
in that flow, it also gets rid of bunch of additional overheads
[eth_get_headlen() etc.] to split headers and data in the skb.

Tested with:
system: 2 sockets, 4 cores per socket, hyperthreading, 2x4x2=16 cores
iperf [server]: iperf -s
iperf [client]: iperf -c <server_ip> -t 500 -i 10 -P 32

HW GRO off – w/o build_skb(), throughput: 36.8 Gbits/sec

Average:     CPU    %usr   %nice    %sys %iowait    %irq   %soft  %steal  %guest   %idle
Average:     all    0.59    0.00   32.93    0.00    0.00   43.07    0.00    0.00   23.42

HW GRO off - with build_skb(), throughput: 36.9 Gbits/sec

Average:     CPU    %usr   %nice    %sys %iowait    %irq   %soft  %steal  %guest   %idle
Average:     all    0.70    0.00   31.70    0.00    0.00   25.68    0.00    0.00   41.92

HW GRO on - w/o build_skb(), throughput: 36.9 Gbits/sec

Average:     CPU    %usr   %nice    %sys %iowait    %irq   %soft  %steal  %guest   %idle
Average:     all    0.86    0.00   24.14    0.00    0.00    6.59    0.00    0.00   68.41

HW GRO on - with build_skb(), throughput: 37.5 Gbits/sec

Average:     CPU    %usr   %nice    %sys %iowait    %irq   %soft  %steal  %guest   %idle
Average:     all    0.87    0.00   23.75    0.00    0.00    6.19    0.00    0.00   69.19
Signed-off-by: Ariel Elior <ariel.elior@cavium.com>
Signed-off-by: Manish Chopra <manish.chopra@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>