This changeset moves ipvlan address under RCU protection, using
a per ipvlan device spinlock to protect list mutation and RCU
read access to protect list traversal.

Also explicitly use RCU read lock to traverse the per port
ipvlans list, so that we can now perform a full address lookup
without asserting the RTNL lock.

Overall this allows the ipvlan driver to check fully for duplicate
addresses - before this commit ipv6 addresses assigned by autoconf
via prefix delegation where accepted without any check - and avoid
the following rntl assertion failure still in the same code path:

 RTNL: assertion failed at drivers/net/ipvlan/ipvlan_core.c (124)
 WARNING: CPU: 15 PID: 0 at drivers/net/ipvlan/ipvlan_core.c:124 ipvlan_addr_busy+0x97/0xa0 [ipvlan]
 Modules linked in: ipvlan(E) ixgbe
 CPU: 15 PID: 0 Comm: swapper/15 Tainted: G            E    4.16.0-rc2.ipvlan+ #1782
 Hardware name: Dell Inc. PowerEdge R730/072T6D, BIOS 2.1.7 06/16/2016
 RIP: 0010:ipvlan_addr_busy+0x97/0xa0 [ipvlan]
 RSP: 0018:ffff881ff9e03768 EFLAGS: 00010286
 RAX: 0000000000000000 RBX: ffff881fdf2a9000 RCX: 0000000000000000
 RDX: 0000000000000001 RSI: 00000000000000f6 RDI: 0000000000000300
 RBP: ffff881fdf2a8000 R08: 0000000000000000 R09: 0000000000000000
 R10: 0000000000000001 R11: ffff881ff9e034c0 R12: ffff881fe07bcc00
 R13: 0000000000000001 R14: ffffffffa02002b0 R15: 0000000000000001
 FS:  0000000000000000(0000) GS:ffff881ff9e00000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 00007fc5c1a4f248 CR3: 000000207e012005 CR4: 00000000001606e0
 Call Trace:
  <IRQ>
  ipvlan_addr6_event+0x6c/0xd0 [ipvlan]
  notifier_call_chain+0x49/0x90
  atomic_notifier_call_chain+0x6a/0x100
  ipv6_add_addr+0x5f9/0x720
  addrconf_prefix_rcv_add_addr+0x244/0x3c0
  addrconf_prefix_rcv+0x2f3/0x790
  ndisc_router_discovery+0x633/0xb70
  ndisc_rcv+0x155/0x180
  icmpv6_rcv+0x4ac/0x5f0
  ip6_input_finish+0x138/0x6a0
  ip6_input+0x41/0x1f0
  ipv6_rcv+0x4db/0x8d0
  __netif_receive_skb_core+0x3d5/0xe40
  netif_receive_skb_internal+0x89/0x370
  napi_gro_receive+0x14f/0x1e0
  ixgbe_clean_rx_irq+0x4ce/0x1020 [ixgbe]
  ixgbe_poll+0x31a/0x7a0 [ixgbe]
  net_rx_action+0x296/0x4f0
  __do_softirq+0xcf/0x4f5
  irq_exit+0xf5/0x110
  do_IRQ+0x62/0x110
  common_interrupt+0x91/0x91
  </IRQ>

 v1 -> v2: drop unneeded in_softirq check in ipvlan_addr6_validator_event()

Fixes: e9997c29 ("ipvlan: fix check for IP addresses in control path")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>