Fixes a NULL pointer derefence bug triggered from tap driver.
When tap_get_user calls virtio_net_hdr_to_skb the skb->dev is null
(in tap.c skb->dev is set after the call to virtio_net_hdr_to_skb)
virtio_net_hdr_to_skb calls dev_parse_header_protocol which
needs skb->dev field to be valid.

The line that trigers the bug is in dev_parse_header_protocol
(dev is at offset 0x10 from skb and is stored in RAX register)
  if (!dev->header_ops || !dev->header_ops->parse_protocol)
  22e1:   mov    0x10(%rbx),%rax
  22e5:	  mov    0x230(%rax),%rax

Setting skb->dev before the call in tap.c fixes the issue.

BUG: kernel NULL pointer dereference, address: 0000000000000230
RIP: 0010:virtio_net_hdr_to_skb.constprop.0+0x335/0x410 [tap]
Code: c0 0f 85 b7 fd ff ff eb d4 41 39 c6 77 cf 29 c6 48 89 df 44 01 f6 e8 7a 79 83 c1 48 85 c0 0f 85 d9 fd ff ff eb b7 48 8b 43 10 <48> 8b 80 30 02 00 00 48 85 c0 74 55 48 8b 40 28 48 85 c0 74 4c 48
RSP: 0018:ffffc90005c27c38 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff888298f25300 RCX: 0000000000000010
RDX: 0000000000000005 RSI: ffffc90005c27cb6 RDI: ffff888298f25300
RBP: ffffc90005c27c80 R08: 00000000ffffffea R09: 00000000000007e8
R10: ffff88858ec77458 R11: 0000000000000000 R12: 0000000000000001
R13: 0000000000000014 R14: ffffc90005c27e08 R15: ffffc90005c27cb6
FS:  0000000000000000(0000) GS:ffff88858ec40000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000230 CR3: 0000000281408006 CR4: 00000000003706e0
Call Trace:
 tap_get_user+0x3f1/0x540 [tap]
 tap_sendmsg+0x56/0x362 [tap]
 ? get_tx_bufs+0xc2/0x1e0 [vhost_net]
 handle_tx_copy+0x114/0x670 [vhost_net]
 handle_tx+0xb0/0xe0 [vhost_net]
 handle_tx_kick+0x15/0x20 [vhost_net]
 vhost_worker+0x7b/0xc0 [vhost]
 ? vhost_vring_call_reset+0x40/0x40 [vhost]
 kthread+0xfa/0x120
 ? kthread_complete_and_exit+0x20/0x20
 ret_from_fork+0x1f/0x30

Fixes: 924a9bc3 ("net: check if protocol extracted by virtio_net_hdr_set_proto is correct")
Signed-off-by: Cezar Bulinaru <cbulinaru@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>