Unit number allocation API

[M. Warner Losh]

In message: <48733.1096570677 at critter.freebsd.dk>
            "Poul-Henning Kamp" <phk at phk.freebsd.dk> writes:
: My personal guess is that driver->attach() and driver->probe() will
: never get out from Giant (I can't seriously see the benefits as
: being bigger than the effort) and therefore I think the problem of
: locking API's like this can be wholesale ignored for a very long
: time.

This is my guess as well.  I have in my tree some locking assertions
to make sure this is the case, and hope to commit them soon.

Warner