more on 750 tbuf faults

[Bruce Nemnich]

I am still getting burned by the translate buffer faults, even after
installing the fix mentioned here recently.  I am losing because I
usually get an mcesr of 0xc or 0xd, indicating a tbuf error *and* a bus
error.  I assume the bus error is due to a junk address from the tbuf,
since I never get one alone.  Therefore, it would seem to be safe to
attempt to restart all tbuf faults (regardless of bus error) after
invalidating the tbuf, since that should retry the memory reference with
an empty tbuf, which should work.

I installed this here, but before posting the diff (it's trivial), I
thought I'd see whether anyone disagrees--I may be braindamaged (I'm
pretty new to vax architecture).
-- 
--Bruce Nemnich, Thinking Machines Corporation, Waltham, MA
  {decvax!cca,ihnp4!mit-eddie,allegra!ias}!godot!bruce, BJN at MIT-MC.ARPA