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/nilfs2/super.c b/fs/nilfs2/super.c
index 1fc9ad3..1099a76 100644
--- a/fs/nilfs2/super.c
+++ b/fs/nilfs2/super.c
@@ -917,9 +917,8 @@
 	if (root->cno == NILFS_CPTREE_CURRENT_CNO) {
 		dentry = d_find_alias(inode);
 		if (!dentry) {
-			dentry = d_alloc_root(inode);
+			dentry = d_make_root(inode);
 			if (!dentry) {
-				iput(inode);
 				ret = -ENOMEM;
 				goto failed_dentry;
 			}