staging/lustre/llite: Close atomic_open race with several openers
Right now, if it's an open of a negative dentry, a race is possible
with several openers who all try to instantiate/rehash the same
dentry and would hit a BUG_ON in d_add.
But in fact if we got a negative dentry in atomic_open, that means
we just revalidated it so no point in talking to MDS at all,
just return ENOENT and make the race go away completely.