On the PCI root bus on the x86 architecture, the risk of hitting
some strange system devices is too high: If a driver isn't loaded,
the resources are not claimed; even if a driver is loaded, it
may not request all resources or even the wrong one. We can neither
trust the rest of the kernel nor ACPI/PNP and CRS parsing to get it
right.

Therefore, explicitly spell out what safeguards we provide, and add
a safeguard to only use resources which are set up exclusively for
the secondary PCI bus (non-subtractive mode): the risk of hitting
system devices is quite low, as they usually aren't connected to
the secondary PCI bus.

CC: Jesse Barnes <jbarnes@virtuousgeek.org>
CC: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: NDominik Brodowski <linux@dominikbrodowski.net>