bcache: free heap cache_set->flush_btree in bch_journal_free

Free the cache_set->flush_bree heap memory on journal free.

Signed-off-by: Wang Sheng-Hui <shhuiw@foxmail.com>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>