fs: Set .opendir to fs_opendir_unsupported for btrfs

Otherwise fs_opendir will fault.

Signed-off-by: Marek Behun <marek.behun@nic.cz>
diff --git a/fs/fs.c b/fs/fs.c
index 84349f3..9c4d67f 100644
--- a/fs/fs.c
+++ b/fs/fs.c
@@ -233,6 +233,7 @@
 		.read = btrfs_read,
 		.write = fs_write_unsupported,
 		.uuid = btrfs_uuid,
+		.opendir = fs_opendir_unsupported,
 	},
 #endif
 	{