Bug 614

	Finally multiple-files-per-tape is working on this tape
	drive, so I started putting 4 dumps per tape.  When I
	went to recover something from the last dump
	(restore ibsf 20 4 /dev/rst0), the "forward space file"
	operation timed out, along with driver complaints about
	"mailbox not free".  Things went downhill rapidly as sd0
	contains important parts of the system, and it was getting
	timeout errors also.  Eventually it rebooted itself.

	10 minutes is not enough time for a forward-space-file, or
	especially multiple forward-space-files.  Writing or reading a 
	full tape (Doesn't seem to vary a lot whether I use 525M, 250M, 
	or 120M tapes, either) takes somewhere in the vicinity of 45 
	minutes to just under an hour.  I suspect forward-space-file 
	is done at the same speed as reading the tape.

	Separate timeouts for "forward space file" and "forward
	space record" might be appropriate.

Fix: Apply the following patch, and rebuild the kernel.
	
How-To-Repeat: 	Create a 6525 tape with multiple dumps on it.  Fill up most
	of the tape, so the beginning of the last dump is well into the
	tape.  

	restore ibsf 20 4 /dev/rst0
	(where 4 represents the number of the last dump).

	With 6525 tapes and my tape drive, a blocking factor of 20 is
	required, according to complaints from the kernel if I forget
	to use it.