btrfs: simplify getting and extracting previous transaction at clean_pinned_extents()

Instead of detecting if there is a previous transaction by comparing the
current transaction's list prev member to the head of the transaction
list (fs_info->trans_list), use the list_is_first() helper which contains
that logic and the naming makes sense since a new transaction is always
added to the end of the list fs_info->trans_list with list_add_tail().

We are also extracting the previous transaction with list_last_entry()
against the transaction, which is correct but confusing because that
function is usually meant to be used against a pointer to the start of a
list and not a member of a list. It is easier to reason by either calling
list_first_entry() against the list fs_info->trans_list, since we can
never have more than two transactions in the list, or by calling
list_prev_entry() against the transaction. So change that to use the later
method.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: fdmanana

fs/btrfs/block-group.c

@@ -1418,9 +1418,8 @@ static bool clean_pinned_extents(struct btrfs_trans_handle *trans,
 	int ret;
 
 	spin_lock(&fs_info->trans_lock);
-	if (trans->transaction->list.prev != &fs_info->trans_list) {
-		prev_trans = list_last_entry(&trans->transaction->list,
-					     struct btrfs_transaction, list);
+	if (!list_is_first(&trans->transaction->list, &fs_info->trans_list)) {
+		prev_trans = list_prev_entry(trans->transaction, list);
 		refcount_inc(&prev_trans->use_count);
 	}
 	spin_unlock(&fs_info->trans_lock);