LU-2317 mgs: both callbacks are to be fixed

The commit ORNL-27 introduced a new lock enqueue which drops a
reference on it in the blocking callbacks. however, it turned out
to be racy as if a lock is taken quickly, callback is called twice,
from enqueue completion and from conflicting lock cancel, and lock
reference is decremented twice.  LU-1259 tried to fix it, but only
for 1 callback.

Fix it for mgs_completion_ast_config as well.

Signed-off-by: Vitaly Fertman <vitaly_fertman@xyratex.com>
Xyratex-bug-id: MRP-792
Change-Id: I49207cfae230318da3b6dcbe3a63c14d8d94a244
Reviewed-on: http://review.whamcloud.com/4744
Tested-by: Hudson
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>