ppp: associate skb with a device at tx

Syzkaller triggered flow dissector warning with the following:

r0 = openat$ppp(0xffffffffffffff9c, &(0x7f0000000000), 0xc0802, 0x0)
ioctl$PPPIOCNEWUNIT(r0, 0xc004743e, &(0x7f00000000c0))
ioctl$PPPIOCSACTIVE(r0, 0x40107446, &(0x7f0000000240)={0x2, &(0x7f0000000180)=[{0x20, 0x0, 0x0, 0xfffff034}, {0x6}]})
pwritev(r0, &(0x7f0000000040)=[{&(0x7f0000000140)='\x00!', 0x2}], 0x1, 0x0, 0x0)

[    9.485814] WARNING: CPU: 3 PID: 329 at net/core/flow_dissector.c:1016 __skb_flow_dissect+0x1ee0/0x1fa0
[    9.485929]  skb_get_poff+0x53/0xa0
[    9.485937]  bpf_skb_get_pay_offset+0xe/0x20
[    9.485944]  ? ppp_send_frame+0xc2/0x5b0
[    9.485949]  ? _raw_spin_unlock_irqrestore+0x40/0x60
[    9.485958]  ? __ppp_xmit_process+0x7a/0xe0
[    9.485968]  ? ppp_xmit_process+0x5b/0xb0
[    9.485974]  ? ppp_write+0x12a/0x190
[    9.485981]  ? do_iter_write+0x18e/0x2d0
[    9.485987]  ? __import_iovec+0x30/0x130
[    9.485997]  ? do_pwritev+0x1b6/0x240
[    9.486016]  ? trace_hardirqs_on+0x47/0x50
[    9.486023]  ? __x64_sys_pwritev+0x24/0x30
[    9.486026]  ? do_syscall_64+0x3d/0x80
[    9.486031]  ? entry_SYSCALL_64_after_hwframe+0x63/0xcd

Flow dissector tries to find skb net namespace either via device
or via socket. Neigher is set in ppp_send_frame, so let's manually
use ppp->dev.

Cc: Paul Mackerras <paulus@samba.org>
Cc: linux-ppp@vger.kernel.org
Reported-by: syzbot+41cab52ab62ee99ed24a@syzkaller.appspotmail.com
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ppp/ppp_generic.c

@@ -1743,6 +1743,8 @@ ppp_send_frame(struct ppp *ppp, struct sk_buff *skb)
 	int len;
 	unsigned char *cp;
 
+	skb->dev = ppp->dev;
+
 	if (proto < 0x8000) {
 #ifdef CONFIG_PPP_FILTER
 		/* check if we should pass this packet */