LU-426 Server failover never finishes

LU-426 Server failover never finishes

Make sure obd_zomebie_impexp_thread get zombie add notification.
It is possible this signal is only get by obd_zombie_barrier, and
barrier gulps this notification and sleeps away and hangs ensues.

Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: Ic4a7b9de19aa5e6a9aaa8fb54e7083629b88119b
Reviewed-on: http://review.whamcloud.com/986
Tested-by: Hudson
Reviewed-by: Jinshan Xiong <jay@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Mikhail Pershin <tappro@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>