When we update 64-bit BARs, we have to perform two config writes.  Between
the writes, the half-written BAR value could match a MEM access intended
for another device.  This could result in corruption of this device (for
writes) or an unexpected response machine check (for reads).

To prevent this, disable MEM decoding while updating such BARs.  This uses
the same safety test as 253d2e54, which disables both MEM and IO while
sizing BARs, namely, we don't disable decoding for host bridge devices.
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>