LU-1592 ldlm: protect obd_export:exp_imp_reverse's change

LU-1592 ldlm: protect obd_export:exp_imp_reverse's change

* Protect obd_export::exp_imp_reverse from reconnect and destroy race.
* Add an assertion in class_import_put() to catch race in the first
  place.

Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: If0abf6717456931c567d8d41c1d20fe49452e959
Reviewed-on: http://review.whamcloud.com/3684
Tested-by: Hudson
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: wangdi <di.wang@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>