PCI: pciehp: Avoid returning prematurely from sysfs requests

A sysfs request to enable or disable a PCIe hotplug slot should not
return before it has been carried out.  That is sought to be achieved by
waiting until the controller's "pending_events" have been cleared.

However the IRQ thread pciehp_ist() clears the "pending_events" before
it acts on them.  If pciehp_sysfs_enable_slot() / _disable_slot() happen
to check the "pending_events" after they have been cleared but while
pciehp_ist() is still running, the functions may return prematurely
with an incorrect return value.

Fix by introducing an "ist_running" flag which must be false before a sysfs
request is allowed to return.

Fixes: 32a8cef2

 ("PCI: pciehp: Enable/disable exclusively from IRQ thread")
Link: https://lore.kernel.org/linux-pci/1562226638-54134-1-git-send-email-wangxiongfeng2@huawei.com
Link: https://lore.kernel.org/r/4174210466e27eb7e2243dd1d801d5f75baaffd8.1565345211.git.lukas@wunner.de
Reported-and-tested-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: stable@vger.kernel.org # v4.19+