LU-8900 mgs: use reference count for fs_db

LU-8900 mgs: use reference count for fs_db

That will prevent the in-using 'fs_db' being freed/erased
by others. Then the user (in subsequent patches) can hold
the 'fs_db' for a long time without holding related lock.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Icf5d548a2c51548aae2c05b1b34f003e725f4e02
Reviewed-on: https://review.whamcloud.com/24415
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>