sit: enable lockless xmits

SIT tunnels can benefit from lockless xmits, using NETIF_F_LLTX

Bench on a 16 cpus machine (dual E5540 cpus), 16 threads sending
10000000 UDP frames via one sit tunnel (size:220 bytes per frame)

Before patch :

real	3m15.399s
user	0m9.185s
sys	51m55.403s

75029.00 87.5% _raw_spin_lock            vmlinux
 1090.00  1.3% dst_release               vmlinux
  902.00  1.1% dev_queue_xmit            vmlinux
  627.00  0.7% sock_wfree                vmlinux
  613.00  0.7% ip6_push_pending_frames   ipv6.ko
  505.00  0.6% __ip_route_output_key     vmlinux

After patch:

real	1m1.387s
user	0m12.489s
sys	15m58.868s

28239.00 23.3% dst_release               vmlinux
13570.00 11.2% ip6_push_pending_frames   ipv6.ko
13118.00 10.8% ip6_append_data           ipv6.ko
 7995.00  6.6% __ip_route_output_key     vmlinux
 7924.00  6.5% sk_dst_check              vmlinux
 5015.00  4.1% udpv6_sendmsg             ipv6.ko
 3594.00  3.0% sock_alloc_send_pskb      vmlinux
 3135.00  2.6% sock_wfree                vmlinux
 3055.00  2.5% ip6_sk_dst_lookup         ipv6.ko
 2473.00  2.0% ip_finish_output          vmlinux

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>