[prevents disk_unbusy panic when disk is loaded (if no
free IOPBs, xdstrategy() would queue the buffer for pickup
by xdcintr() but xdcintr() would never call disk_busy().
xdc_startbuf() is a better place since all bufs are routed
through here] problem detected by girish@dworkin.wustl.edu,
diagnosed and corrected by me.
- move disk_unbusy() call in xdc_remove_iorq() before the call to
XDC_FREE() [don't want to access a data structure that was just put
on a free list]
2accbfeefb