Btrfs: tests: checking for NULL instead of IS_ERR()

btrfs_alloc_dummy_root() return an error pointer on failure, it never
returns NULL.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/tests/free-space-tests.c

@@ -898,8 +898,10 @@ int btrfs_test_free_space_cache(void)
 	}
 
 	root = btrfs_alloc_dummy_root();
-	if (!root)
+	if (IS_ERR(root)) {
+		ret = PTR_ERR(root);
 		goto out;
+	}
 
 	root->fs_info = btrfs_alloc_dummy_fs_info();
 	if (!root->fs_info)