esp: remove the skb from the chain when it's enqueued in cryptd_wq

esp: remove the skb from the chain when it's enqueued in cryptd_wq

Xiumei found a panic in esp offload:

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000020
  RIP: 0010:esp_output_done+0x101/0x160 [esp4]
  Call Trace:
   ? esp_output+0x180/0x180 [esp4]
   cryptd_aead_crypt+0x4c/0x90
   cryptd_queue_worker+0x6e/0xa0
   process_one_work+0x1a7/0x3b0
   worker_thread+0x30/0x390
   ? create_worker+0x1a0/0x1a0
   kthread+0x112/0x130
   ? kthread_flush_work_fn+0x10/0x10
   ret_from_fork+0x35/0x40

It was caused by that skb secpath is used in esp_output_done() after it's
been released elsewhere.

The tx path for esp offload is:

  __dev_queue_xmit()->
    validate_xmit_skb_list()->
      validate_xmit_xfrm()->
        esp_xmit()->
          esp_output_tail()->
            aead_request_set_callback(esp_output_done) <--[1]
            crypto_aead_encrypt()  <--[2]

In [1], .callback is set, and in [2] it will trigger the worker schedule,
later on a kernel thread will call .callback(esp_output_done), as the call
trace shows.

But in validate_xmit_xfrm():

  skb_list_walk_safe(skb, skb2, nskb) {
    ...
    err = x->type_offload->xmit(x, skb2, esp_features);  [esp_xmit]
    ...
  }

When the err is -EINPROGRESS, which means this skb2 will be enqueued and
later gets encrypted and sent out by .callback later in a kernel thread,
skb2 should be removed fromt skb chain. Otherwise, it will get processed
again outside validate_xmit_xfrm(), which could release skb secpath, and
cause the panic above.

This patch is to remove the skb from the chain when it's enqueued in
cryptd_wq. While at it, remove the unnecessary 'if (!skb)' check.

Fixes: 3dca3f38cfb8 ("xfrm: Separate ESP handling from segmentation for GRO packets.")
Reported-by: Xiumei Mu <xmu@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>