LU-1046 ldlm: hold ldlm_cb_set_arg until the last user exit

LU-1046 ldlm: hold ldlm_cb_set_arg until the last user exit

There is race condition between ldlm_cb_interpret() and
ldlm_run_ast_work() when test/release "ldlm_cb_set_arg":
ldlm_run_ast_work() can exit before ldlm_cb_interpret()
processing "ldlm_cb_set_arg::waitq", then causes invalid
memory accessing.

Signed-off-by: Fan Yong <yong.fan@whamcloud.com>
Change-Id: Idfb3f41739e02a79fe4310ecbc7bb842e75a82d8
Reviewed-on: http://review.whamcloud.com/2065
Tested-by: Hudson
Reviewed-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>