PCI: Add device disconnected state

PCI: Add device disconnected state

Add a new state to pci_dev to be set when it is unexpectedly disconnected.
The PCI driver tear down functions can observe this new device state so
they may skip operations that will fail.

The pciehp and pcie-dpc drivers are aware when the link is down, so these
set the flag when their handlers detect the device is disconnected.

Tested-by: Krishna Dhulipala <[email protected]>
Signed-off-by: Keith Busch <[email protected]>
Signed-off-by: Bjorn Helgaas <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Wei Zhang <[email protected]>