LU-2104 recovery: keep valid obd_stale_clients during recovery

LU-2104 recovery: keep valid obd_stale_clients during recovery

- in class_fail_export() update obd_stale_clients counter if
  recovery is in progress. That fixes endless recovery issues.
- class_disconnect_stale_exports() should check exp_failed and
  exclude such exports from being evicted once more.
- set obd_no_conn flag in ofd_prepare() like mdt does.
- remove check for obd_no_conn from check_for_clients(). That is
  extra check as we don't allow any client to connect while
  obd_no_conn is set

Signed-off-by: Mikhail Pershin <tappro@whamcloud.com>
Change-Id: I439a1edd49a9c51f42df4c423d7c165468a3d38d
Reviewed-on: http://review.whamcloud.com/4636
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>