netfilter: br_netfilter: fix panic with metadata_dst skb
In the Linux kernel, the following vulnerability has been resolved:

netfilter: br_netfilter: fix panic with metadata_dst skb

Fix a kernel panic in the br_netfilter module when sending untagged
traffic via a VxLAN device.
This happens during the check for fragmentation in br_nf_dev_queue_xmit.

It is dependent on:
1) the br_netfilter module being loaded;
2) net.bridge.bridge-nf-call-iptables set to 1;
3) a bridge with a VxLAN (single-vxlan-device) netdevice as a bridge port;
4) untagged frames with size higher than the VxLAN MTU forwarded/flooded

When forwarding the untagged packet to the VxLAN bridge port, before
the netfilter hooks are called, br_handle_egress_vlan_tunnel is called and
changes the skb_dst to the tunnel dst. The tunnel_dst is a metadata type
of dst, i.e., skb_valid_dst(skb) is false, and metadata->dst.dev is NULL.

Then in the br_netfilter hooks, in br_nf_dev_queue_xmit, there's a check
for frames that needs to be fragmented: frames with higher MTU than the
VxLAN device end up calling br_nf_ip_fragment, which in turns call
ip_skb_dst_mtu.

The ip_dst_mtu tries to use the skb_dst(skb) as if it was a valid dst
with valid dst->dev, thus the crash.

This case was never supported in the first place, so drop the packet
instead.

PING 10.0.0.2 (10.0.0.2) from 0.0.0.0 h1-eth0: 2000(2028) bytes of data.
[  176.291791] Unable to handle kernel NULL pointer dereference at
virtual address 0000000000000110
[  176.292101] Mem abort info:
[  176.292184]   ESR = 0x0000000096000004
[  176.292322]   EC = 0x25: DABT (current EL), IL = 32 bits
[  176.292530]   SET = 0, FnV = 0
[  176.292709]   EA = 0, S1PTW = 0
[  176.292862]   FSC = 0x04: level 0 translation fault
[  176.293013] Data abort info:
[  176.293104]   ISV = 0, ISS = 0x00000004, ISS2 = 0x00000000
[  176.293488]   CM = 0, WnR = 0, TnD = 0, TagAccess = 0
[  176.293787]   GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
[  176.293995] user pgtable: 4k pages, 48-bit VAs, pgdp=0000000043ef5000
[  176.294166] [0000000000000110] pgd=0000000000000000,
p4d=0000000000000000
[  176.294827] Internal error: Oops: 0000000096000004 [#1] PREEMPT SMP
[  176.295252] Modules linked in: vxlan ip6_udp_tunnel udp_tunnel veth
br_netfilter bridge stp llc ipv6 crct10dif_ce
[  176.295923] CPU: 0 PID: 188 Comm: ping Not tainted
6.8.0-rc3-g5b3fbd61b9d1 #2
[  176.296314] Hardware name: linux,dummy-virt (DT)
[  176.296535] pstate: 80000005 (Nzcv daif -PAN -UAO -TCO -DIT -SSBS
BTYPE=--)
[  176.296808] pc : br_nf_dev_queue_xmit+0x390/0x4ec [br_netfilter]
[  176.297382] lr : br_nf_dev_queue_xmit+0x2ac/0x4ec [br_netfilter]
[  176.297636] sp : ffff800080003630
[  176.297743] x29: ffff800080003630 x28: 0000000000000008 x27:
ffff6828c49ad9f8
[  176.298093] x26: ffff6828c49ad000 x25: 0000000000000000 x24:
00000000000003e8
[  176.298430] x23: 0000000000000000 x22: ffff6828c4960b40 x21:
ffff6828c3b16d28
[  176.298652] x20: ffff6828c3167048 x19: ffff6828c3b16d00 x18:
0000000000000014
[  176.298926] x17: ffffb0476322f000 x16: ffffb7e164023730 x15:
0000000095744632
[  176.299296] x14: ffff6828c3f1c880 x13: 0000000000000002 x12:
ffffb7e137926a70
[  176.299574] x11: 0000000000000001 x10: ffff6828c3f1c898 x9 :
0000000000000000
[  176.300049] x8 : ffff6828c49bf070 x7 : 0008460f18d5f20e x6 :
f20e0100bebafeca
[  176.300302] x5 : ffff6828c7f918fe x4 : ffff6828c49bf070 x3 :
0000000000000000
[  176.300586] x2 : 0000000000000000 x1 : ffff6828c3c7ad00 x0 :
ffff6828c7f918f0
[  176.300889] Call trace:
[  176.301123]  br_nf_dev_queue_xmit+0x390/0x4ec [br_netfilter]
[  176.301411]  br_nf_post_routing+0x2a8/0x3e4 [br_netfilter]
[  176.301703]  nf_hook_slow+0x48/0x124
[  176.302060]  br_forward_finish+0xc8/0xe8 [bridge]
[  176.302371]  br_nf_hook_thresh+0x124/0x134 [br_netfilter]
[  176.302605]  br_nf_forward_finish+0x118/0x22c [br_netfilter]
[  176.302824]  br_nf_forward_ip.part.0+0x264/0x290 [br_netfilter]
[  176.303136]  br_nf_forward+0x2b8/0x4e0 [br_netfilter]
[  176.303359]  nf_hook_slow+0x48/0x124
[  176.303
---truncated---