Use the new per task frag allocator in skb_append_datato_frags(),
to reduce number of frags and page allocator overhead.

Tested:
 ifconfig lo mtu 16436
 perf record netperf -t UDP_STREAM ; perf report

before :
 Throughput: 32928 Mbit/s
    51.79%  netperf  [kernel.kallsyms]  [k] copy_user_generic_string
     5.98%  netperf  [kernel.kallsyms]  [k] __alloc_pages_nodemask
     5.58%  netperf  [kernel.kallsyms]  [k] get_page_from_freelist
     5.01%  netperf  [kernel.kallsyms]  [k] __rmqueue
     3.74%  netperf  [kernel.kallsyms]  [k] skb_append_datato_frags
     1.87%  netperf  [kernel.kallsyms]  [k] prep_new_page
     1.42%  netperf  [kernel.kallsyms]  [k] next_zones_zonelist
     1.28%  netperf  [kernel.kallsyms]  [k] __inc_zone_state
     1.26%  netperf  [kernel.kallsyms]  [k] alloc_pages_current
     0.78%  netperf  [kernel.kallsyms]  [k] sock_alloc_send_pskb
     0.74%  netperf  [kernel.kallsyms]  [k] udp_sendmsg
     0.72%  netperf  [kernel.kallsyms]  [k] zone_watermark_ok
     0.68%  netperf  [kernel.kallsyms]  [k] __cpuset_node_allowed_softwall
     0.67%  netperf  [kernel.kallsyms]  [k] fib_table_lookup
     0.60%  netperf  [kernel.kallsyms]  [k] memcpy_fromiovecend
     0.55%  netperf  [kernel.kallsyms]  [k] __udp4_lib_lookup

 after:
  Throughput: 47185 Mbit/s
	61.74%	netperf  [kernel.kallsyms]	[k] copy_user_generic_string
	 2.07%	netperf  [kernel.kallsyms]	[k] prep_new_page
	 1.98%	netperf  [kernel.kallsyms]	[k] skb_append_datato_frags
	 1.02%	netperf  [kernel.kallsyms]	[k] sock_alloc_send_pskb
	 0.97%	netperf  [kernel.kallsyms]	[k] enqueue_task_fair
	 0.97%	netperf  [kernel.kallsyms]	[k] udp_sendmsg
	 0.91%	netperf  [kernel.kallsyms]	[k] __ip_route_output_key
	 0.88%	netperf  [kernel.kallsyms]	[k] __netif_receive_skb
	 0.87%	netperf  [kernel.kallsyms]	[k] fib_table_lookup
	 0.85%	netperf  [kernel.kallsyms]	[k] resched_task
	 0.78%	netperf  [kernel.kallsyms]	[k] __udp4_lib_lookup
	 0.77%	netperf  [kernel.kallsyms]	[k] _raw_spin_lock_irqsave

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>