new helper: lookup_positive_unlocked()

Most of the callers of lookup_one_len_unlocked() treat negatives are
ERR_PTR(-ENOENT).  Provide a helper that would do just that.  Note
that a pinned positive dentry remains positive - it's ->d_inode is
stable, etc.; a pinned _negative_ dentry can become positive at any
point as long as you are not holding its parent at least shared.
So using lookup_one_len_unlocked() needs to be careful;
lookup_positive_unlocked() is safer and that's what the callers
end up open-coding anyway.

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

fs/kernfs/mount.c

@@ -200,7 +200,7 @@ struct dentry *kernfs_node_dentry(struct kernfs_node *kn,
 			dput(dentry);
 			return ERR_PTR(-EINVAL);
 		}
-		dtmp = lookup_one_len_unlocked(kntmp->name, dentry,
+		dtmp = lookup_positive_unlocked(kntmp->name, dentry,
 					       strlen(kntmp->name));
 		dput(dentry);
 		if (IS_ERR(dtmp))