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

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

drivers/misc/ibmasm/ibmasmfs.c

@@ -129,11 +129,9 @@ static int ibmasmfs_fill_super (struct super_block *sb, void *data, int silent)
 	root->i_op = &simple_dir_inode_operations;
 	root->i_fop = ibmasmfs_dir_ops;
 
-	root_dentry = d_alloc_root(root);
-	if (!root_dentry) {
-		iput(root);
+	root_dentry = d_make_root(root);
+	if (!root_dentry)
 		return -ENOMEM;
-	}
 	sb->s_root = root_dentry;
 
 	ibmasmfs_create_files(sb, root_dentry);