Re: biodone panics

Take 2 for the biodone() panics:

acd_geom_start() implements request limiting by breaking up a large
request in the passed bio into a sequence of smaller ones.  As each
request is created,  acd_strategy is invoked to start the IO.

However, I think this IO can complete while still issuing the child
requests, leading to the parent being retired early. (ie, when a child
operation completes, it checks if its the last operation to complete,
and, if so, retires the parent: see g_std_done.)

The attached patch makes my qemu box much more reliable (I could crash
a qemu hosted system 100% with a "tar fc" of the 6.0-BETA bootonly ISO
without the patch, and it's gone through many iterations fine with it)

Any opinions/testing results welcome.