reiserfs: Fix use after free in journal teardown

reiserfs: Fix use after free in journal teardown

commit 01777836c87081e4f68c4a43c9abe6114805f91e upstream.

If do_journal_release() races with do_journal_end() which requeues
delayed works for transaction flushing, we can leave work items for
flushing outstanding transactions queued while freeing them. That
results in use after free and possible crash in run_timers_softirq().

Fix the problem by not requeueing works if superblock is being shut down
(MS_ACTIVE not set) and using cancel_delayed_work_sync() in
do_journal_release().

Signed-off-by: Jan Kara <jack@suse.cz>
[bwh: Backported to 3.2:
 - Adjust context
 - commit_wq is global, not per-superblock
 - Change comment about 'these works'; we only have one work item
 - Drop inapplicable changes to reiserfs_schedule_old_flush()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>