PCI: Add schedule point in proc_bus_pci_read()

PCI configuration space reads from /proc/bus/pci can often take several
milliseconds to complete.

Add a schedule point in proc_bus_pci_read() to reduce the maximum latency.

A similar change was made for sysfs by 2ce02a86 ("PCI: Add schedule
point in pci_read_config()").

Link: https://lore.kernel.org/r/20200824052025.48362-1-benbjiang@tencent.com
Link: https://lore.kernel.org/r/20210815150824.96773-1-kw@linux.com


Signed-off-by: Krzysztof Wilczyński <kw@linux.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>