make sure do_dentry_open() won't return positive as an error

An ->open() instances really, really should not be doing that.  There's
a lot of places e.g. around atomic_open() that could be confused by that,
so let's catch that early.
Acked-by: NLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

fs/open.c

@@ -812,6 +812,8 @@ static int do_dentry_open(struct file *f,
 	return 0;
 
 cleanup_all:
+	if (WARN_ON_ONCE(error > 0))
+		error = -EINVAL;
 	fops_put(f->f_op);
 	if (f->f_mode & FMODE_WRITER) {
 		put_write_access(inode);