Push Giant down one layer

The /dev/pci device doesn't need GIANT, per se. However, one routine
that it calls, pci_find_dbsf implicitly does. It walks a list that can
change when PCI scans a new bus. With hotplug, this means we could
have a race with that scanning. To prevent that, take out Giant around
scanning the list.

However, given that we have places in the tree that drop giant, if
held when we call into them, the whole use of Giant to protect newbus
may be less effective that we desire, so add a comment about why we're
talking it out, and we'll address the issue when we lock newbus with
something other than Giant.