net: fix NULL pointer in skb_segment_list

Commit 3a1296a3 ("net: Support GRO/GSO fraglist chaining.")
introduced UDP listifyed GRO. The segmentation relies on frag_list being
untouched when passing through the network stack. This assumption can be
broken sometimes, where frag_list itself gets pulled into linear area,
leaving frag_list being NULL. When this happens it can trigger
following NULL pointer dereference, and panic the kernel. Reverse the
test condition should fix it.

[19185.577801][    C1] BUG: kernel NULL pointer dereference, address:
...
[19185.663775][    C1] RIP: 0010:skb_segment_list+0x1cc/0x390
...
[19185.834644][    C1] Call Trace:
[19185.841730][    C1]  <TASK>
[19185.848563][    C1]  __udp_gso_segment+0x33e/0x510
[19185.857370][    C1]  inet_gso_segment+0x15b/0x3e0
[19185.866059][    C1]  skb_mac_gso_segment+0x97/0x110
[19185.874939][    C1]  __skb_gso_segment+0xb2/0x160
[19185.883646][    C1]  udp_queue_rcv_skb+0xc3/0x1d0
[19185.892319][    C1]  udp_unicast_rcv_skb+0x75/0x90
[19185.900979][    C1]  ip_protocol_deliver_rcu+0xd2/0x200
[19185.910003][    C1]  ip_local_deliver_finish+0x44/0x60
[19185.918757][    C1]  __netif_receive_skb_one_core+0x8b/0xa0
[19185.927834][    C1]  process_backlog+0x88/0x130
[19185.935840][    C1]  __napi_poll+0x27/0x150
[19185.943447][    C1]  net_rx_action+0x27e/0x5f0
[19185.951331][    C1]  ? mlx5_cq_tasklet_cb+0x70/0x160 [mlx5_core]
[19185.960848][    C1]  __do_softirq+0xbc/0x25d
[19185.968607][    C1]  irq_exit_rcu+0x83/0xb0
[19185.976247][    C1]  common_interrupt+0x43/0xa0
[19185.984235][    C1]  asm_common_interrupt+0x22/0x40
...
[19186.094106][    C1]  </TASK>

Fixes: 3a1296a3 ("net: Support GRO/GSO fraglist chaining.")
Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Yan Zhai <yan@cloudflare.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/Y9gt5EUizK1UImEP@debian


Signed-off-by: Jakub Kicinski <kuba@kernel.org>