PCI: hv: Lock PCI bus on device eject

A PCI_EJECT message can arrive at the same time we are calling
pci_scan_child_bus() in the workqueue for the previous PCI_BUS_RELATIONS
message or in create_root_hv_pci_bus().  In this case we could potentially
modify the bus from multiple places.

Properly lock the bus access.

Thanks Dexuan Cui <decui@microsoft.com> for pointing out the race condition
in create_root_hv_pci_bus().

Reported-by: Xiaofeng Wang <xiaofwan@redhat.com>
Signed-off-by: Long Li <longli@microsoft.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: K. Y. Srinivasan <kys@microsoft.com>