reiserfs: Make cancel_old_flush() reliable

Currently canceling of delayed work that flushes old data using
cancel_old_flush() does not prevent work from being requeued. Thus
in theory new work can be queued after cancel_old_flush() from
reiserfs_freeze() has run. This will become larger problem once
flush_old_commits() can requeue the work itself.

Fix the problem by recording in sbi->work_queue that flushing work is
canceled and should not be requeued.

Signed-off-by: Jan Kara <jack@suse.cz>