btrfs: simplify getting and extracting previous transaction during commit

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().

And instead of extracting the previous transaction with the more generic
list_entry() helper against the current transaction's list prev member,
use the more specific list_prev_entry() helper, which makes it clear what
we are doing and is shorter.

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>