Merge branch 'ipv6_dev_get_saddr-rcu'

Eric Dumazet says:

====================
ipv6: ipv6_dev_get_saddr() rcu works

Sending IPv6 udp packets on non connected sockets is quite slow,
because ipv6_dev_get_saddr() is still using an rwlock and silly
references games on ifa.

Tested:

$ ./super_netperf 16 -H 4444::555:0786 -l 2000 -t UDP_STREAM -- -m 100 &
[1] 12527

Performance is boosted from 2.02 Mpps to 4.28 Mpps

Kernel profile before patches :
  22.62%  [kernel]  [k] _raw_read_lock_bh
   7.04%  [kernel]  [k] refcount_sub_and_test
   6.56%  [kernel]  [k] ipv6_get_saddr_eval
   5.67%  [kernel]  [k] _raw_read_unlock_bh
   5.34%  [kernel]  [k] __ipv6_dev_get_saddr
   4.95%  [kernel]  [k] refcount_inc_not_zero
   4.03%  [kernel]  [k] __ip6addrlbl_match
   3.70%  [kernel]  [k] _raw_spin_lock
   3.44%  [kernel]  [k] ipv6_dev_get_saddr
   3.24%  [kernel]  [k] ip6_pol_route
   3.06%  [kernel]  [k] refcount_add_not_zero
   2.30%  [kernel]  [k] __local_bh_enable_ip
   1.81%  [kernel]  [k] mlx4_en_xmit
   1.20%  [kernel]  [k] __ip6_append_data
   1.12%  [kernel]  [k] __ip6_make_skb
   1.11%  [kernel]  [k] __dev_queue_xmit
   1.06%  [kernel]  [k] l3mdev_master_ifindex_rcu

Kernel profile after patches :
  11.36%  [kernel]  [k] ip6_pol_route
   7.65%  [kernel]  [k] _raw_spin_lock
   7.16%  [kernel]  [k] __ipv6_dev_get_saddr
   6.49%  [kernel]  [k] ipv6_get_saddr_eval
   6.04%  [kernel]  [k] refcount_add_not_zero
   3.34%  [kernel]  [k] __ip6addrlbl_match
   2.62%  [kernel]  [k] __dev_queue_xmit
   2.37%  [kernel]  [k] mlx4_en_xmit
   2.26%  [kernel]  [k] dst_release
   1.89%  [kernel]  [k] __ip6_make_skb
   1.87%  [kernel]  [k] __ip6_append_data
   1.86%  [kernel]  [k] udpv6_sendmsg
   1.86%  [kernel]  [k] ip6t_do_table
   1.64%  [kernel]  [k] ipv6_dev_get_saddr
   1.64%  [kernel]  [k] find_match
   1.51%  [kernel]  [k] l3mdev_master_ifindex_rcu
   1.24%  [kernel]  [k] ipv6_addr_label
====================

Signed-off-by: David S. Miller <davem@davemloft.net>