btrfs: remove unnecessary cur_trans set before goto loop in join_transaction

In the big loop, cur_trans will be set fs_info->running_transaction
before it's used. And after kmem_cache_free it and goto loop, it will
be setup again. No need to setup it immediately after freed.

Signed-off-by: Wang Sheng-Hui <shhuiw@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>