LU-5573 obdclass: strengthen against concurrent server mounts

LU-5573 obdclass: strengthen against concurrent server mounts

This patch is an add-on to previous fix for LU-5299, against same
issues/races scenarios during concurrent server devices mounts.

Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: Ib12332e9758f4a64fb8d7bdf60464100b4fd6c00
Reviewed-on: http://review.whamcloud.com/12114
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>