cifs: warn and fail if trying to use rootfs without the config option

If CONFIG_CIFS_ROOT is not set, rootfs mount option is invalid
Signed-off-by: Aurelien Aptel <aaptel@suse.com>
CC: <stable@vger.kernel.org> # v5.11
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/fs_context.c

@@ -1196,9 +1196,11 @@ static int smb3_fs_context_parse_param(struct fs_context *fc,
 		pr_warn_once("Witness protocol support is experimental\n");
 		break;
 	case Opt_rootfs:
-#ifdef CONFIG_CIFS_ROOT
-		ctx->rootfs = true;
+#ifndef CONFIG_CIFS_ROOT
+		cifs_dbg(VFS, "rootfs support requires CONFIG_CIFS_ROOT config option\n");
+		goto cifs_parse_mount_err;
 #endif
+		ctx->rootfs = true;
 		break;
 	case Opt_posixpaths:
 		if (result.negated)