Create wrapper for Giant taken for newbus

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