Bug 17374

The wt QIC02 tapedriver in thwe FreeBSD 3.x has a nice bug (could be
also in NetBSD and other BSD's ???)
The wt driver sometimes comes in a deadlock state (waiting for an 
wake-up event that never occurs). It may happen when you are doing 
longer backups on - I assume - some "slow" PC's like mine. 
The backup does not finish correctly, it "hangs".
The code is in /sys/i386/isa/wt.c.
The solution is simple. After changing it,
the bug never occurred (to me) again. 

Reason:
In the Interrupt routine wtintr() there is no provision for 
stopping the timeout() if all i/o (one block) is completed
(state 'i/o finished'). If an interrupt is generated at this moment,
it comes in an unexpected state ('continue i/o'). In this state,
there is of course no wakeup, so it keeps sleeping ... 

Patch:
*Allways put the timer off* if i/o is finished.
Add this line in the function body of wtintr(sc):
	untimeout(wtimer, (caddr_t)t, t->co_handler);
See context-diff below.
Remark:
In NetBSD ther is no call-out handler, the patch is even
more simple.

How-To-Repeat: 
Problem shows up every time I was making a longer backup, 
on my old 486 PC system.
(May be on a fast system it does not occure, because it is a timing problem)