xfrm: policy: fix metadata dst->dev xmit null pointer dereference

In the Linux kernel, the following vulnerability has been resolved:

xfrm: policy: fix metadata dst->dev xmit null pointer dereference

When we try to transmit an skb with metadata_dst attached (i.e. dst->dev
== NULL) through xfrm interface we can hit a null pointer dereference[1]
in xfrmi_xmit2() -> xfrm_lookup_with_ifid() due to the check for a
loopback skb device when there's no policy which dereferences dst->dev
unconditionally. Not having dst->dev can be interepreted as it not being
a loopback device, so just add a check for a null dst_orig->dev.

With this fix xfrm interface's Tx error counters go up as usual.

[1] net-next calltrace captured via netconsole:
BUG: kernel NULL pointer dereference, address: 00000000000000c0
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
PGD 0 P4D 0
Oops: 0000 [#1] PREEMPT SMP
CPU: 1 PID: 7231 Comm: ping Kdump: loaded Not tainted 5.19.0+ #24
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.0-1.fc36 04/01/2014
RIP: 0010:xfrm_lookup_with_ifid+0x5eb/0xa60
Code: 8d 74 24 38 e8 26 a4 37 00 48 89 c1 e9 12 fc ff ff 49 63 ed 41 83 fd be 0f 85 be 01 00 00 41 be ff ff ff ff 45 31 ed 48 8b 03 <f6> 80 c0 00 00 00 08 75 0f 41 80 bc 24 19 0d 00 00 01 0f 84 1e 02
RSP: 0018:ffffb0db82c679f0 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffffd0db7fcad430 RCX: ffffb0db82c67a10
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffb0db82c67a80
RBP: ffffb0db82c67a80 R08: ffffb0db82c67a14 R09: 0000000000000000
R10: 0000000000000000 R11: ffff8fa449667dc8 R12: ffffffff966db880
R13: 0000000000000000 R14: 00000000ffffffff R15: 0000000000000000
FS: 00007ff35c83f000(0000) GS:ffff8fa478480000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000000000c0 CR3: 000000001ebb7000 CR4: 0000000000350ee0
Call Trace:
<TASK>
xfrmi_xmit+0xde/0x460
? tcf_bpf_act+0x13d/0x2a0
dev_hard_start_xmit+0x72/0x1e0
__dev_queue_xmit+0x251/0xd30
ip_finish_output2+0x140/0x550
ip_push_pending_frames+0x56/0x80
raw_sendmsg+0x663/0x10a0
? try_charge_memcg+0x3fd/0x7a0
? __mod_memcg_lruvec_state+0x93/0x110
? sock_sendmsg+0x30/0x40
sock_sendmsg+0x30/0x40
__sys_sendto+0xeb/0x130
? handle_mm_fault+0xae/0x280
? do_user_addr_fault+0x1e7/0x680
? kvm_read_and_reset_apf_flags+0x3b/0x50
__x64_sys_sendto+0x20/0x30
do_syscall_64+0x34/0x80
entry_SYSCALL_64_after_hwframe+0x46/0xb0
RIP: 0033:0x7ff35cac1366
Code: eb 0b 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b8 0f 1f 00 41 89 ca 64 8b 04 25 18 00 00 00 85 c0 75 11 b8 2c 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 72 c3 90 55 48 83 ec 30 44 89 4c 24 2c 4c 89
RSP: 002b:00007fff738e4028 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
RAX: ffffffffffffffda RBX: 00007fff738e57b0 RCX: 00007ff35cac1366
RDX: 0000000000000040 RSI: 0000557164e4b450 RDI: 0000000000000003
RBP: 0000557164e4b450 R08: 00007fff738e7a2c R09: 0000000000000010
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000040
R13: 00007fff738e5770 R14: 00007fff738e4030 R15: 0000001d00000001
</TASK>
Modules linked in: netconsole veth br_netfilter bridge bonding virtio_net [last unloaded: netconsole]
CR2: 00000000000000c0