subflow_add_reset_reason(skb, ...) can fail.

We can not assume mptcp_get_ext(skb) always return a non NULL pointer.

syzbot reported:

general protection fault, probably for non-canonical address 0xdffffc0000000003: 0000 [#1] PREEMPT SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000018-0x000000000000001f]
CPU: 0 PID: 5098 Comm: syz-executor132 Not tainted 6.9.0-rc6-syzkaller-01478-gcdc74c9d #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/27/2024
 RIP: 0010:subflow_v6_route_req+0x2c7/0x490 net/mptcp/subflow.c:388
Code: 8d 7b 07 48 89 f8 48 c1 e8 03 42 0f b6 04 20 84 c0 0f 85 c0 01 00 00 0f b6 43 07 48 8d 1c c3 48 83 c3 18 48 89 d8 48 c1 e8 03 <42> 0f b6 04 20 84 c0 0f 85 84 01 00 00 0f b6 5b 01 83 e3 0f 48 89
RSP: 0018:ffffc9000362eb68 EFLAGS: 00010206
RAX: 0000000000000003 RBX: 0000000000000018 RCX: ffff888022039e00
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffff88807d961140 R08: ffffffff8b6cb76b R09: 1ffff1100fb2c230
R10: dffffc0000000000 R11: ffffed100fb2c231 R12: dffffc0000000000
R13: ffff888022bfe273 R14: ffff88802cf9cc80 R15: ffff88802ad5a700
FS:  0000555587ad2380(0000) GS:ffff8880b9400000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f420c3f9720 CR3: 0000000022bfc000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
  tcp_conn_request+0xf07/0x32c0 net/ipv4/tcp_input.c:7180
  tcp_rcv_state_process+0x183c/0x4500 net/ipv4/tcp_input.c:6663
  tcp_v6_do_rcv+0x8b2/0x1310 net/ipv6/tcp_ipv6.c:1673
  tcp_v6_rcv+0x22b4/0x30b0 net/ipv6/tcp_ipv6.c:1910
  ip6_protocol_deliver_rcu+0xc76/0x1570 net/ipv6/ip6_input.c:438
  ip6_input_finish+0x186/0x2d0 net/ipv6/ip6_input.c:483
  NF_HOOK+0x3a4/0x450 include/linux/netfilter.h:314
  NF_HOOK+0x3a4/0x450 include/linux/netfilter.h:314
  __netif_receive_skb_one_core net/core/dev.c:5625 [inline]
  __netif_receive_skb+0x1ea/0x650 net/core/dev.c:5739
  netif_receive_skb_internal net/core/dev.c:5825 [inline]
  netif_receive_skb+0x1e8/0x890 net/core/dev.c:5885
  tun_rx_batched+0x1b7/0x8f0 drivers/net/tun.c:1549
  tun_get_user+0x2f35/0x4560 drivers/net/tun.c:2002
  tun_chr_write_iter+0x113/0x1f0 drivers/net/tun.c:2048
  call_write_iter include/linux/fs.h:2110 [inline]
  new_sync_write fs/read_write.c:497 [inline]
  vfs_write+0xa84/0xcb0 fs/read_write.c:590
  ksys_write+0x1a0/0x2c0 fs/read_write.c:643
  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
  do_syscall_64+0xf5/0x240 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Fixes: 3e140491 ("mptcp: support rstreason for passive reset")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Link: https://lore.kernel.org/r/20240506123032.3351895-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>