btrfs: remove unused btrfs_root::defrag_trans_start

Last touched in 2013 by commit de78b51a ("btrfs: remove cache only
arguments from defrag path") that was the only code that used the value.
Now it's only set but never used for anything, so we can remove it.
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -1075,7 +1075,6 @@ struct btrfs_root {
 
 	u64 highest_objectid;
 
-	u64 defrag_trans_start;
 	struct btrfs_key defrag_progress;
 	struct btrfs_key defrag_max;
 

fs/btrfs/disk-io.c

@@ -1141,10 +1141,6 @@ static void __setup_root(struct btrfs_root *root, struct btrfs_fs_info *fs_info,
 	memset(&root->root_key, 0, sizeof(root->root_key));
 	memset(&root->root_item, 0, sizeof(root->root_item));
 	memset(&root->defrag_progress, 0, sizeof(root->defrag_progress));
-	if (!dummy)
-		root->defrag_trans_start = fs_info->generation;
-	else
-		root->defrag_trans_start = 0;
 	root->root_key.objectid = objectid;
 	root->anon_dev = 0;
 

fs/btrfs/tree-defrag.c

@@ -133,10 +133,9 @@ int btrfs_defrag_leaves(struct btrfs_trans_handle *trans,
 		ret = 0;
 	}
 done:
-	if (ret != -EAGAIN) {
+	if (ret != -EAGAIN)
 		memset(&root->defrag_progress, 0,
 		       sizeof(root->defrag_progress));
-		root->defrag_trans_start = trans->transid;
-	}
+
 	return ret;
 }