LU-4772 mgs: check MGS refcounting before export barrier

LU-4772 mgs: check MGS refcounting before export barrier

Patch adds debug code to make sure that there are no extra
MGS export references taken before export barrier.

Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Change-Id: I4a7ae3130cb70a9b6847c9d4b5c2874ccf9ee72d
Reviewed-on: http://review.whamcloud.com/13920
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>