PCI: endpoint: pci-epf-test: Handle Link Down event

Per PCIe r6.0, sec 5.2, a Link Down event can happen under any of the
following circumstances:

  1. Fundamental/Hot reset
  2. Link disable transmission by upstream component
  3. Moving from L2/L3 to L0

When the event happens, the EPC driver capable of detecting it may pass the
notification to the EPF driver through link_down() callback in 'struct
pci_epc_event_ops'.

While the PCIe spec has not defined the actual behavior of the endpoint
when the Link Down event happens, we may assume that at least the ongoing
transactions need to be stopped as the link won't be active, so
cancel the command handler work in the callback implementation
pci_epf_test_link_down(). The work will be started again in
pci_epf_test_link_up() once the link comes back again.

Link: https://lore.kernel.org/linux-pci/[email protected]
Tested-by: Niklas Cassel <[email protected]>
Signed-off-by: Manivannan Sadhasivam <[email protected]>
Signed-off-by: Krzysztof Wilczyński <[email protected]>
[bhelgaas: update spec citation]
Signed-off-by: Bjorn Helgaas <[email protected]>
Reviewed-by: Niklas Cassel <[email protected]>

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