Create a wrapper for newbus to take giant and for busses to take it too.
bus_topo_lock() should be called before interacting with newbus routines
and unlocked with bus_topo_unlock(). If you need the topology lock for
some reason, bus_topo_mtx() will provide that.
Sponsored by: Netflix
Reviewed by: mav
Differential Revision: https://reviews.freebsd.org/D31831
(cherry picked from commit c6df6f5322)
