openvswitch: Fix ovs_vport_get_stats()

Not every device has dev->tstats set. So when OVS tries to calculate
vport stats it causes kernel panic. Following patch fixes it by
using standard API to get net-device stats.

---8<---
Unable to handle kernel paging request at virtual address 766b4008
Internal error: Oops: 96000005 [#1] PREEMPT SMP
Modules linked in: vport_vxlan vxlan ip6_udp_tunnel udp_tunnel tun bridge stp llc openvswitch ipv6
CPU: 7 PID: 1108 Comm: ovs-vswitchd Not tainted 4.3.0-rc3+ #82
PC is at ovs_vport_get_stats+0x150/0x1f8 [openvswitch]
<snip>
Call trace:
 [<ffffffbffc0859f8>] ovs_vport_get_stats+0x150/0x1f8 [openvswitch]
 [<ffffffbffc07cdb0>] ovs_vport_cmd_fill_info+0x140/0x1e0 [openvswitch]
 [<ffffffbffc07cf0c>] ovs_vport_cmd_dump+0xbc/0x138 [openvswitch]
 [<ffffffc00045a5ac>] netlink_dump+0xb8/0x258
 [<ffffffc00045ace0>] __netlink_dump_start+0x120/0x178
 [<ffffffc00045dd9c>] genl_family_rcv_msg+0x2d4/0x308
 [<ffffffc00045de58>] genl_rcv_msg+0x88/0xc4
 [<ffffffc00045cf24>] netlink_rcv_skb+0xd4/0x100
 [<ffffffc00045dab0>] genl_rcv+0x30/0x48
 [<ffffffc00045c830>] netlink_unicast+0x154/0x200
 [<ffffffc00045cc9c>] netlink_sendmsg+0x308/0x364
 [<ffffffc00041e10c>] sock_sendmsg+0x14/0x2c
 [<ffffffc000420d58>] SyS_sendto+0xbc/0xf0
Code: aa1603e1 f94037a4 aa1303e2 aa1703e0 (f9400465)

Reported-by: Tomasz Sawicki <tomasz.sawicki@objectiveintegration.uk>
Fixes: 8c876639 ("openvswitch: Remove vport stats.")
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>