LU-7221 ldlm: do not take a reference on target if stopping

LU-7221 ldlm: do not take a reference on target if stopping

In the set of changes of patch for LU-5569
(http://review.whamcloud.com/11750/,
commit 892078e3b566c04471e7dcf2c28e66f2f3584f93) one is to take a
reference on target even if it is stopping (umount'ed). Then, upon
connections attempts, this can lead to unwanted cleanup actions to
occur on [obd_self_]export from class_decref(), finally causing a
LBUG in class_export_put() because export's exp_refcount has already
reached 0.

Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: If960437934fb694d173a4fd1fbfb9e43d496fea6
Reviewed-on: http://review.whamcloud.com/16940
Tested-by: Jenkins
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>