PCI: Fix race condition with driver_override

The driver_override implementation is susceptible to a race condition when
different threads are reading vs. storing a different driver override.  Add
locking to avoid the race condition.

This is in close analogy to commit 6265539776a0 ("driver core: platform:
fix race condition with driver_override") from Adrian Salido.

Change-Id: Ic5ddbe28ba71c4e45892bb1a6bb1d81d3b34a33a
Fixes: 782a985d ("PCI: Introduce new device binding path using pci_dev.driver_override")
Signed-off-by: Nicolai Stange <nstange@suse.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: stable@vger.kernel.org	# v3.16+
Git-commit: 9561475db680f7144d2223a409dd3d7e322aca03
Git-repo: https://android.googlesource.com/kernel/common


Signed-off-by: Srinivasa Rao Kuppala <srkupp@codeaurora.org>