nullfs stabilization I

[Simon 'corecode' Schubert]

Matthew Dillon wrote:
    (2) Do not have a shadow callback.  Instead have null_nresolve()
	call a new function, cache_shadow(), which creates the relationship
	between the upper and lower layer namecache records.  Only
	null_nresolve() would need the added sophistication.
While I completely agree with the rest of your proposals, I see one 
problem here:

Imagine you create a file in the NULLFS layer.  For this the code 
cache_lookup()s and then gets a locked namecache entry, which might not 
be resolved yet (new file, not in cache).  This would mean that we have 
a locked record but no shadow association.  At the same time another 
process can race in the shadowed layer and as well lock the record which 
is the counterpart to the already locked NULLFS namecache entry.

Not good.  This is the reason why I used this callback in the first 
place.  Do you have a better method to do this?

cheers
  simon
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