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

CC: Steffen Klassert <steffen.klassert@secunet.com>
CC: Daniel Borkmann <daniel@iogearbox.net>
Fixes: 2d151d39 ("xfrm: Add possibility to set the default to block if we have no policy")
Signed-off-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>