PCI: Add schedule point in pci_read_config()

PCI: Add schedule point in pci_read_config()

The PCI sysfs "config" file allows large reads, and the resulting PCI
config reads can take several milliseconds to complete.  Testing with the
cyclictest [1] benchmark showed 5ms+ latencies.

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

[1] https://git.kernel.org/pub/scm/linux/kernel/git/clrkwllms/rt-tests.git/

[bhelgaas: commit log]
Link: https://lore.kernel.org/r/20200824052025.48362-1-benbjiang@tencent.com
Reported-by: Bin Lai <robinlai@tencent.com>
Signed-off-by: Jiang Biao <benbjiang@tencent.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>