flow_dissector: Update pptp handling to avoid null pointer deref.

flow_dissector: Update pptp handling to avoid null pointer deref.

__skb_flow_dissect can be called with a skb or a data packet, either
can be NULL. All calls seems to have been moved to __skb_header_pointer
except the pptp handling which is still calling skb_header_pointer.

skb_header_pointer will use skb->data and thus:
[  109.556866] BUG: unable to handle kernel NULL pointer dereference at 0000000000000080
[  109.557102] IP: [<ffffffff88dc02f8>] __skb_flow_dissect+0xa88/0xce0
[  109.557263] PGD 0
[  109.557338]
[  109.557484] Oops: 0000 [#1] SMP
[  109.557562] Modules linked in: chaoskey
[  109.557783] CPU: 2 PID: 0 Comm: swapper/2 Not tainted 4.9.0 #79
[  109.557867] Hardware name: Supermicro A1SRM-LN7F/LN5F/A1SRM-LN7F-2758, BIOS 1.0c 11/04/2015
[  109.557957] task: ffff94085c27bc00 task.stack: ffffb745c0068000
[  109.558041] RIP: 0010:[<ffffffff88dc02f8>]  [<ffffffff88dc02f8>] __skb_flow_dissect+0xa88/0xce0
[  109.558203] RSP: 0018:ffff94087fc83d40  EFLAGS: 00010206
[  109.558286] RAX: 0000000000000130 RBX: ffffffff8975bf80 RCX: ffff94084fab6800
[  109.558373] RDX: 0000000000000010 RSI: 000000000000000c RDI: 0000000000000000
[  109.558460] RBP: 0000000000000b88 R08: 0000000000000000 R09: 0000000000000022
[  109.558547] R10: 0000000000000008 R11: ffff94087fc83e04 R12: 0000000000000000
[  109.558763] R13: ffff94084fab6800 R14: ffff94087fc83e04 R15: 000000000000002f
[  109.558979] FS:  0000000000000000(0000) GS:ffff94087fc80000(0000) knlGS:0000000000000000
[  109.559326] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  109.559539] CR2: 0000000000000080 CR3: 0000000281809000 CR4: 00000000001026e0
[  109.559753] Stack:
[  109.559957]  000000000000000c ffff94084fab6822 0000000000000001 ffff94085c2b5fc0
[  109.560578]  0000000000000001 0000000000002000 0000000000000000 0000000000000000
[  109.561200]  0000000000000000 0000000000000000 0000000000000000 0000000000000000
[  109.561820] Call Trace:
[  109.562027]  <IRQ>
[  109.562108]  [<ffffffff88dfb4fa>] ? eth_get_headlen+0x7a/0xf0
[  109.562522]  [<ffffffff88c5a35a>] ? igb_poll+0x96a/0xe80
[  109.562737]  [<ffffffff88dc912b>] ? net_rx_action+0x20b/0x350
[  109.562953]  [<ffffffff88546d68>] ? __do_softirq+0xe8/0x280
[  109.563169]  [<ffffffff8854704a>] ? irq_exit+0xaa/0xb0
[  109.563382]  [<ffffffff8847229b>] ? do_IRQ+0x4b/0xc0
[  109.563597]  [<ffffffff8902d4ff>] ? common_interrupt+0x7f/0x7f
[  109.563810]  <EOI>
[  109.563890]  [<ffffffff88d57530>] ? cpuidle_enter_state+0x130/0x2c0
[  109.564304]  [<ffffffff88d57520>] ? cpuidle_enter_state+0x120/0x2c0
[  109.564520]  [<ffffffff8857eacf>] ? cpu_startup_entry+0x19f/0x1f0
[  109.564737]  [<ffffffff8848d55a>] ? start_secondary+0x12a/0x140
[  109.564950] Code: 83 e2 20 a8 80 0f 84 60 01 00 00 c7 04 24 08 00
00 00 66 85 d2 0f 84 be fe ff ff e9 69 fe ff ff 8b 34 24 89 f2 83 c2
04 66 85 c0 <41> 8b 84 24 80 00 00 00 0f 49 d6 41 8d 31 01 d6 41 2b 84
24 84
[  109.569959] RIP  [<ffffffff88dc02f8>] __skb_flow_dissect+0xa88/0xce0
[  109.570245]  RSP <ffff94087fc83d40>
[  109.570453] CR2: 0000000000000080

Fixes: ab10dccb1160 ("rps: Inspect PPTP encapsulated by GRE to get flow hash")
Signed-off-by: Ian Kumlien <ian.kumlien@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>