firmware loader: fix pending_fw_head list corruption

Got the following oops just before reboot:

Unable to handle kernel NULL pointer dereference at virtual address 00000000
[<8028d300>] (__list_del_entry+0x44/0xac)
[<802e3320>] (__fw_load_abort.part.13+0x1c/0x50)
[<802e337c>] (fw_shutdown_notify+0x28/0x50)
[<80034f80>] (notifier_call_chain.isra.1+0x5c/0x9c)
[<800350ec>] (__blocking_notifier_call_chain+0x44/0x58)
[<80035114>] (blocking_notifier_call_chain+0x14/0x18)
[<80035d64>] (kernel_restart_prepare+0x14/0x38)
[<80035d94>] (kernel_restart+0xc/0x50)

The following race condition triggers here:

  _request_firmware_load()
  device_create_file(...)
  kobject_uevent(...)
  (schedule)
                                       (resume)
                                       firmware_loading_store(1)
                                       firmware_loading_store(0)
                                       list_del_init(&buf->pending_list)
                                       (schedule)
  (resume)
  list_add(&buf->pending_list, &pending_fw_head);
  wait_for_completion(&buf->completion);

causing an oops later when walking pending_list after the firmware has
been released.

The proposed fix is to move the list_add() before sysfs attribute
creation.

Signed-off-by: Maxime Bizon <mbizon@freebox.fr>
Acked-by: Ming Lei <ming.lei@canonical.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>