switch open-coded instances of d_make_root() to new helper

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/isofs/inode.c b/fs/isofs/inode.c
index bd62c76..29037c3 100644
--- a/fs/isofs/inode.c
+++ b/fs/isofs/inode.c
@@ -947,9 +947,8 @@
 	s->s_d_op = &isofs_dentry_ops[table];
 
 	/* get the root dentry */
-	s->s_root = d_alloc_root(inode);
+	s->s_root = d_make_root(inode);
 	if (!(s->s_root)) {
-		iput(inode);
 		error = -ENOMEM;
 		goto out_no_inode;
 	}