datapath: stt: Use RCU API to update stt-dev list.

datapath: stt: Use RCU API to update stt-dev list.

Following crash was reported for STT tunnel. I am not able to reproduce
it, But the usage of wrong list manipulation API is likely culprit.

---8<---
IP: [<ffffffffc0e731fd>] nf_ip_hook+0xfd/0x180 [openvswitch]
Oops: 0000 [#1] PREEMPT SMP
Hardware name: VMware, Inc. VMware Virtual Platform/440BX
RIP: 0010:[<ffffffffc0e731fd>]  [<ffffffffc0e731fd>] nf_ip_hook+0xfd/0x180 [openvswitch]
RSP: 0018:ffff88043fd03cd0  EFLAGS: 00010206
RAX: 0000000000000000 RBX: ffff8801008e2200 RCX: 0000000000000034
RDX: 0000000000000110 RSI: ffff8801008e2200 RDI: ffff8801533a3880
RBP: ffff88043fd03d00 R08: ffffffff90646d10 R09: ffff880164b27000
R10: 0000000000000003 R11: ffff880155eb9dd8 R12: 0000000000000028
R13: ffff8802283dc580 R14: 00000000000076b4 R15: ffff880013b20000
FS:  00007ff5ba73b700(0000) GS:ffff88043fd00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000020 CR3: 000000037ff96000 CR4: 00000000000007e0
Stack:
 ffff8801533a3890 ffff88043fd03d80 ffffffff90646d10 0000000000000000
 ffff880164b27000 ffff8801008e2200 ffff88043fd03d48 ffffffff9064050a
 ffffffff90d0f930 ffffffffc0e7ef80 0000000000000001 ffff8801008e2200
Call Trace:
 <IRQ>
 [<ffffffff90646d10>] ? ip_rcv_finish+0x350/0x350
 [<ffffffff9064050a>] nf_iterate+0x9a/0xb0
 [<ffffffff90646d10>] ? ip_rcv_finish+0x350/0x350
 [<ffffffff9064059c>] nf_hook_slow+0x7c/0x120
 [<ffffffff90646d10>] ? ip_rcv_finish+0x350/0x350
 [<ffffffff906470f3>] ip_local_deliver+0x73/0x80
 [<ffffffff90646a3d>] ip_rcv_finish+0x7d/0x350
 [<ffffffff90647398>] ip_rcv+0x298/0x3d0
 [<ffffffff9060fc56>] __netif_receive_skb_core+0x696/0x880
 [<ffffffff9060fe58>] __netif_receive_skb+0x18/0x60
 [<ffffffff90610b3e>] process_backlog+0xae/0x180
 [<ffffffff906102c2>] net_rx_action+0x152/0x270
 [<ffffffff9006d625>] __do_softirq+0xf5/0x320
 [<ffffffff9071d15c>] do_softirq_own_stack+0x1c/0x30

Reported-by: Joe Stringer <joe@ovn.org>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Jesse Gross <jesse@kernel.org>