vfio/pci: Separate SR-IOV VF dev_set

In the below noted Fixes commit we introduced a reflck mutex to allow
better scaling between devices for open and close.  The reflck was
based on the hot reset granularity, device level for root bus devices
which cannot support hot reset or bus/slot reset otherwise.  Overlooked
in this were SR-IOV VFs, where there's also no bus reset option, but
the default for a non-root-bus, non-slot-based device is bus level
reflck granularity.

The reflck mutex has since become the dev_set mutex (via commit
2cd8b14aaa ("vfio/pci: Move to the device set infrastructure")) and
is our defacto serialization for various operations and ioctls.  It
still seems to be the case though that sets of vfio-pci devices really
only need serialization relative to hot resets affecting the entire
set, which is not relevant to SR-IOV VFs.  As described in the Closes
link below, this serialization contributes to startup latency when
multiple VFs sharing the same "bus" are opened concurrently.

Mark the device itself as the basis of the dev_set for SR-IOV VFs.

Reported-by: Aaron Lewis <aaronlewis@google.com>
Closes: https://lore.kernel.org/all/20250626180424.632628-1-aaronlewis@google.com
Tested-by: Aaron Lewis <aaronlewis@google.com>
Fixes: e309df5b0c ("vfio/pci: Parallelize device open and release")
Reviewed-by: Yi Liu <yi.l.liu@intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20250626225623.1180952-1-alex.williamson@redhat.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/pci/vfio_pci_core.c

@@ -2149,7 +2149,7 @@ int vfio_pci_core_register_device(struct vfio_pci_core_device *vdev)
 		return -EBUSY;
 	}
 
-	if (pci_is_root_bus(pdev->bus)) {
+	if (pci_is_root_bus(pdev->bus) || pdev->is_virtfn) {
 		ret = vfio_assign_device_set(&vdev->vdev, vdev);
 	} else if (!pci_probe_reset_slot(pdev->slot)) {
 		ret = vfio_assign_device_set(&vdev->vdev, pdev->slot);