ovl: force read-only sb on failure to create index dir

With index feature enabled, on failure to create index dir, overlay is
being mounted read-only.  However, we do not forbid user to remount overlay
read-write.  Fix that by setting ofs->workdir to NULL, which prevents
remount read-write.
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/super.c

@@ -1352,12 +1352,13 @@ static int ovl_get_indexdir(struct super_block *sb, struct ovl_fs *ofs,
 		goto out;
 	}
 
+	/* index dir will act also as workdir */
+	iput(ofs->workdir_trap);
+	ofs->workdir_trap = NULL;
+	dput(ofs->workdir);
+	ofs->workdir = NULL;
 	ofs->indexdir = ovl_workdir_create(ofs, OVL_INDEXDIR_NAME, true);
 	if (ofs->indexdir) {
-		/* index dir will act also as workdir */
-		iput(ofs->workdir_trap);
-		ofs->workdir_trap = NULL;
-		dput(ofs->workdir);
 		ofs->workdir = dget(ofs->indexdir);
 
 		err = ovl_setup_trap(sb, ofs->indexdir, &ofs->indexdir_trap,
@@ -1869,7 +1870,7 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 	if (!ovl_upper_mnt(ofs))
 		sb->s_flags |= SB_RDONLY;
 
-	if (!(ovl_force_readonly(ofs)) && ofs->config.index) {
+	if (!ovl_force_readonly(ofs) && ofs->config.index) {
 		err = ovl_get_indexdir(sb, ofs, oe, &upperpath);
 		if (err)
 			goto out_free_oe;