datapath: Restore OVS_CB after skb_segment.

OVS needs to segments large skb before sending it for miss
packet handling to userspace. but skb_gso_segment uses
skb->cb. This corrupted OVS_CB which result in following panic.

[  735.419921] BUG: unable to handle kernel paging request at 00000014000001b2
[  735.423168] Oops: 0000 [#1] SMP DEBUG_PAGEALLOC
[  735.445097] RIP: 0010:[<ffffffffa05df0d7>]  [<ffffffffa05df0d7>] ovs_nla_put_flow+0x37/0x7c0 [openvswitch]
[  735.468858] Call Trace:
[  735.470384]  [<ffffffffa05d7ec2>] queue_userspace_packet+0x332/0x4d0 [openvswitch]
[  735.471741]  [<ffffffffa05d8155>] queue_gso_packets+0xf5/0x180 [openvswitch]
[  735.481862]  [<ffffffffa05da9f5>] ovs_dp_upcall+0x65/0x70 [openvswitch]
[  735.483031]  [<ffffffffa05dab81>] ovs_dp_process_packet+0x181/0x1b0 [openvswitch]
[  735.484391]  [<ffffffffa05e2f55>] ovs_vport_receive+0x65/0x90 [openvswitch]
[  735.492638]  [<ffffffffa05e5738>] internal_dev_xmit+0x68/0x110 [openvswitch]
[  735.495334]  [<ffffffff81588eb6>] dev_hard_start_xmit+0x2e6/0x8b0
[  735.496503]  [<ffffffff81589847>] __dev_queue_xmit+0x3c7/0x920
[  735.499827]  [<ffffffff81589db0>] dev_queue_xmit+0x10/0x20
[  735.500798]  [<ffffffff815d3b60>] ip_finish_output+0x6a0/0x950
[  735.502818]  [<ffffffff815d55f8>] ip_output+0x68/0x110
[  735.503835]  [<ffffffff815d4979>] ip_local_out+0x29/0x90
[  735.504801]  [<ffffffff815d4e46>] ip_queue_xmit+0x1d6/0x640
[  735.507015]  [<ffffffff815ee0d7>] tcp_transmit_skb+0x477/0xac0
[  735.508260]  [<ffffffff815ee856>] tcp_write_xmit+0x136/0xba0
[  735.510829]  [<ffffffff815ef56e>] __tcp_push_pending_frames+0x2e/0xc0
[  735.512296]  [<ffffffff815e0593>] tcp_sendmsg+0xa63/0xd50
[  735.513526]  [<ffffffff81612c2c>] inet_sendmsg+0x10c/0x220
[  735.516025]  [<ffffffff81566b8c>] sock_sendmsg+0x9c/0xe0
[  735.518066]  [<ffffffff81566d41>] SYSC_sendto+0x121/0x1c0
[  735.521398]  [<ffffffff8156801e>] SyS_sendto+0xe/0x10
[  735.522473]  [<ffffffff816df5e9>] system_call_fastpath+0x16/0x1b

Reported-by: Andy Zhou <azhou@nicira.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Andy Zhou <azhou@nicira.com>