PCI/ASPM: Disable ASPM on MFD function removal to avoid use-after-free

PCI/ASPM: Disable ASPM on MFD function removal to avoid use-after-free

Struct pcie_link_state->downstream is a pointer to the pci_dev of function
0.  Previously we retained that pointer when removing function 0, and
subsequent ASPM policy changes dereferenced it, resulting in a
use-after-free warning from KASAN, e.g.:

  # echo 1 > /sys/bus/pci/devices/0000:03:00.0/remove
  # echo powersave > /sys/module/pcie_aspm/parameters/policy

  BUG: KASAN: slab-use-after-free in pcie_config_aspm_link+0x42d/0x500
  Call Trace:
   kasan_report+0xae/0xe0
   pcie_config_aspm_link+0x42d/0x500
   pcie_aspm_set_policy+0x8e/0x1a0
   param_attr_store+0x162/0x2c0
   module_attr_store+0x3e/0x80

PCIe spec r6.0, sec 7.5.3.7, recommends that software program the same ASPM
Control value in all functions of multi-function devices.

Disable ASPM and free the pcie_link_state when any child function is
removed so we can discard the dangling pcie_link_state->downstream pointer
and maintain the same ASPM Control configuration for all functions.

[bhelgaas: commit log and comment]
Debugged-by: Zongquan Qin <qinzongquan@sangfor.com.cn>
Suggested-by: Bjorn Helgaas <bhelgaas@google.com>
Fixes: b5a0a9b59c81 ("PCI/ASPM: Read and set up L1 substate capabilities")
Link: https://lore.kernel.org/r/20230507034057.20970-1-dinghui@sangfor.com.cn
Signed-off-by: Ding Hui <dinghui@sangfor.com.cn>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>