MFC 233662,233677-233678,250418,252166:

MFC 233662,233677-233678,250418,252166:
If the firmware/BIOS assigns conflicting ranges to BARs then leaving the
BARs alone could result in one device stealing mmio accesses intended to go
to a second device.  Previously the PCI bus driver attempted to handle this
case by clearing the BAR to 0 depending on BARs based at 0 not decoding (which
is not guaranteed to be true).  Now when a conflicting BAR is detected the
following steps are taken:

 1) If hw.pci.realloc_bars (a new tunable) is enabled (default is disabled),
    then ignore the current BAR setting from the firmware and attempt to
    allocate a fresh resource range for the BAR.

 2) If 1) failed (or was disabled), disable decoding for the relevant
    BAR type (e.g. disable mem decoding for a memory BAR) and emit a
    warning if booting verbose.

git-svn-id: svn://svn.freebsd.org/base/stable/8@252318 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f