btrfs: constify remaining structs with function pointers

* struct extent_io_ops
* struct btrfs_free_space_op

Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/tests/free-space-tests.c b/fs/btrfs/tests/free-space-tests.c
index 46c2f29..69a11e6 100644
--- a/fs/btrfs/tests/free-space-tests.c
+++ b/fs/btrfs/tests/free-space-tests.c
@@ -445,11 +445,11 @@
 	int ret;
 	u64 offset;
 	u64 max_extent_size;
-	struct btrfs_free_space_op test_free_space_ops = {
+	const struct btrfs_free_space_op test_free_space_ops = {
 		.recalc_thresholds = cache->free_space_ctl->op->recalc_thresholds,
 		.use_bitmap = test_use_bitmap,
 	};
-	struct btrfs_free_space_op *orig_free_space_ops;
+	const struct btrfs_free_space_op *orig_free_space_ops;
 
 	test_msg("Running space stealing from bitmap to extent\n");