reiserfs: fix race with flush_used_journal_lists and flush_journal_list

There are two locks involved in managing the journal lists. The general
reiserfs_write_lock and the journal->j_flush_mutex.

While flush_journal_list is sleeping to acquire the j_flush_mutex or to
submit a block for write, it will drop the write lock. This allows
another thread to acquire the write lock and ultimately call
flush_used_journal_lists to traverse the list of journal lists and
select one for flushing. It can select the journal_list that has just
had flush_journal_list called on it in the original thread and call it
again with the same journal_list.

The second thread then drops the write lock to acquire j_flush_mutex and
the first thread reacquires it and continues execution and eventually
clears and frees the journal list before dropping j_flush_mutex and
returning.

The second thread acquires j_flush_mutex and ends up operating on a
journal_list that has already been released. If the memory hasn't
been reused, we'll soon...