PCI: Don't update VF BARs while VF memory space is enabled

If we update a VF BAR while it's enabled, there are two potential problems:

  1) Any driver that's using the VF has a cached BAR value that is stale
     after the update, and

  2) We can't update 64-bit BARs atomically, so the intermediate state
     (new lower dword with old upper dword) may conflict with another
     device, and an access by a driver unrelated to the VF may cause a bus
     error.

Warn about attempts to update VF BARs while they are enabled.  This is a
programming error, so use dev_WARN() to get a backtrace.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Gavin Shan <gwshan@linux.vnet.ibm.com>