PCI: Only allow WC mmap on prefetchable resources

commit cef4d02305a06be581bb7f4353446717a1b319ec upstream.

The /proc/bus/pci mmap interface allows the user to specify whether they
want WC or not.  Don't let them do so on non-prefetchable BARs.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>