ocfs2/dlm: fix misuse of list_move_tail() in dlm_run_purge_list()

When a lockres in purge list but is still in use, it should be moved to
the tail of purge list.  dlm_thread will continue to check next lockres in
purge list.  However, code list_move_tail(&dlm->purge_list,
&lockres->purge) will do *no* movements, so dlm_thread will purge the same
lockres in this loop again and again.  If it is in use for a long time,
other lockres will not be processed.

Signed-off-by: Yiwen Jiang <jiangyiwen@huawei.com>
Signed-off-by: joyce.xue <xuejiufei@huawei.com>
Reviewed-by: Mark Fasheh <mfasheh@suse.de>
Cc: Joel Becker <jlbec@evilplan.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>