some changes to xpt to help make unloading sim modules safer

[Chuck Tuffli]

On Sun, Aug 7, 2011 at 2:27 PM, Matthew Jacob <mj at feral.com> wrote:
>
> See http://people.freebsd.org/~mjacob/xpt_patches.txt

Matt -

Thanks for doing this. So it seems the rule is to hold
xsoftc.xpt_topo_lock while modifying bus->refcount . If so, does
xpt_alloc_target need mtx_lock() / mtx_unlock() around bus->refcount++
?

---chuck