PCI: pciehp: Avoid slot access during reset

The ->reset_slot callback introduced by commits:

  2e35afae ("PCI: pciehp: Add reset_slot() method") and
  06a8d89a

 ("PCI: pciehp: Disable link notification across slot reset")

disables notification of Presence Detect Changed and Data Link Layer
State Changed events for the duration of a secondary bus reset.

However a bus reset not only triggers these events, but may also clear
the Presence Detect State bit in the Slot Status register and the Data
Link Layer Link Active bit in the Link Status register momentarily.
According to Sinan Kaya:

 "I know for a fact that bus reset clears the Data Link Layer Active bit
  as soon as link goes down.  It gets set again following link up.
  Presence detect depends on the HW implementation.  QDT root ports
  don't change presence detect for instance since nobody actually
  removed the card.  If an implementation supports in-band presence
  detect, the answer is yes.  As soon as the link goes down, presence
  detect bit will get cleared until recovery."
  https://lkml.kernel.org/r/42e72f83-3b24-f7ef-e5bc-290fae99259a@codeaurora.org

  In-band presence detect is also covered in Table 4-15 in PCIe r4.0,
  sec 4.2.6.

pciehp should therefore ensure that any parts of the driver that access
those bits do not run concurrently to a bus reset.  The only precaution
the commits took to that effect was to halt interrupt polling.  They
made no effort to drain the slot workqueue, cancel an outstanding
Attention Button work, or block slot enable/disable requests via sysfs
and in the ->probe hook.

Now that pciehp is converted to enable/disable the slot exclusively from
the IRQ thread, the only places accessing the two above-mentioned bits
are the IRQ thread and the ->probe hook.  Add locking to serialize them
with a bus reset.  This obviates the need to halt interrupt polling.
Do not add locking to the ->get_adapter_status sysfs callback to afford
users unfettered access to that bit.  Use an rw_semaphore in lieu of a
regular mutex to allow parallel execution of the non-reset code paths
accessing the critical bits, i.e. the IRQ thread and the ->probe hook.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Rajat Jain <rajatja@google.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Sinan Kaya <okaya@kernel.org>