PCI: vmd: Move SRCU cleanup after bus, child device removal

Recent __call_srcu() changes have exposed that we need to cleanup SRCU
structures after pci_stop_root_bus() calls into vmd_msi_free().

Fixes: 3906b918 ("PCI: vmd: Use SRCU as a local RCU to prevent delaying global RCU")
Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Keith Busch <keith.busch@intel.com>
Cc: <stable@vger.kernel.org> # 4.11