beginning of transition to parallel lookups - marking in-lookup dentries

marked as such when (would be) parallel lookup is about to pass them
to actual ->lookup(); unmarked when
	* __d_add() is about to make it hashed, positive or not.
	* __d_move() (from d_splice_alias(), directly or via
__d_unalias()) puts a preexisting dentry in its place
	* in caller of ->lookup() if it has escaped all of the
above.  Bug (WARN_ON, actually) if it reaches the final dput()
or d_instantiate() while still marked such.

As the result, we are guaranteed that for as long as the flag is
set, dentry will
	* remain negative unhashed with positive refcount
	* never have its ->d_alias looked at
	* never have its ->d_lru looked at
	* never have its ->d_parent and ->d_name changed

Right now we have at most one such for any given parent directory.
With parallel lookups that restriction will weaken to
	* only exist when parent is locked shared
	* at most one with given (parent,name) pair (comparison of
names is according to ->d_compare())
	* only exist when there's no hashed dentry with the same
(parent,name)

Transition will take the next several commits; unfortunately, we'll
only be able to switch to rwsem at the end of this series.  The
reason for not making it a single patch is to simplify review.

New primitives: d_in_lookup() (a predicate checking if dentry is in
the in-lookup state) and d_lookup_done() (tells the system that
we are done with lookup and if it's still marked as in-lookup, it
should cease to be such).

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namei.c

@@ -1634,7 +1634,11 @@ static struct dentry *lookup_slow(const struct qstr *name,
 		inode_unlock(inode);
 		return ERR_PTR(-ENOMEM);
 	}
+	spin_lock(&dentry->d_lock);
+	dentry->d_flags |= DCACHE_PAR_LOOKUP;
+	spin_unlock(&dentry->d_lock);
 	old = inode->i_op->lookup(inode, dentry, flags);
+	d_lookup_done(dentry);
 	if (unlikely(old)) {
 		dput(dentry);
 		dentry = old;