Btrfs: remove no longer used member num_dirty_bgs from transaction

The member num_dirty_bgs of struct btrfs_transaction is not used anymore,
it is set and incremented but nothing reads its value anymore. Its last
read use was removed by commit 64403612 ("btrfs: rework
btrfs_check_space_for_delayed_refs"). So just remove that member.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -6431,7 +6431,6 @@ static int update_block_group(struct btrfs_trans_handle *trans,
 		if (list_empty(&cache->dirty_list)) {
 			list_add_tail(&cache->dirty_list,
 				      &trans->transaction->dirty_bgs);
-			trans->transaction->num_dirty_bgs++;
 			trans->delayed_ref_updates++;
 			btrfs_get_block_group(cache);
 		}

fs/btrfs/transaction.c

@@ -233,7 +233,6 @@ static noinline int join_transaction(struct btrfs_fs_info *fs_info,
 	INIT_LIST_HEAD(&cur_trans->io_bgs);
 	INIT_LIST_HEAD(&cur_trans->dropped_roots);
 	mutex_init(&cur_trans->cache_write_mutex);
-	cur_trans->num_dirty_bgs = 0;
 	spin_lock_init(&cur_trans->dirty_bgs_lock);
 	INIT_LIST_HEAD(&cur_trans->deleted_bgs);
 	spin_lock_init(&cur_trans->dropped_roots_lock);

fs/btrfs/transaction.h

@@ -80,7 +80,6 @@ struct btrfs_transaction {
 	 */
 	struct mutex cache_write_mutex;
 	spinlock_t dirty_bgs_lock;
-	unsigned int num_dirty_bgs;
 	/* Protected by spin lock fs_info->unused_bgs_lock. */
 	struct list_head deleted_bgs;
 	spinlock_t dropped_roots_lock;