LU-395: obd_zombie_barrier is not barrier actually

LU-395: obd_zombie_barrier is not barrier actually

a hotfix for avoiding deadlock:
sys_umount -> class_detach -> obd_zombie_impexp_cull -> filter_cleanup
-> obd_zombie_barrier, but the zombies_count will only be decreased after
obd_zombie_impexp_cull, so this process is waiting for itself, this is a
deadlock.

Signed-off-by: Jinshan Xiong <jay@whamcloud.com>
Change-Id: Id29deab4c85a7570ec59b2a20f70ccfae9440f01
Reviewed-on: http://review.whamcloud.com/945
Reviewed-by: Mikhail Pershin <tappro@whamcloud.com>
Tested-by: Hudson
Reviewed-by: Oleg Drokin <green@whamcloud.com>