LU-4772 mgs: free MGS fsdb before export barrier

LU-4772 mgs: free MGS fsdb before export barrier

The mgs_fsc keeps export reference and should be cleaned up
before the waiting for exports cleanup. Patch moves FSDB cleanup
functions which cleanups also mgs_fsc ahead of obd_export_barried()

Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Change-Id: I98aa2e669a7ed2cba3b31df44fd3e96dcbbb6807
Reviewed-on: http://review.whamcloud.com/14443
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>