Pull up following revision(s) (requested by hannken in ticket #781):
sys/kern/kern_physio.c: revision 1.91
PR kern/39536: bufq related problem when writing DVDR and DVDRWs.
Remove a race where physio_done() may use memory already freed.
Observed by Hans Rosenfeld <rosenfeld@grumpf.hope-2000.org>.

(snj)