LU-8900 snapshot: new config for MDT write barrier

LU-8900 snapshot: new config for MDT write barrier

We need some mechanism to set write barrier on the MDT, with that,
all the subsequent modification will be blocked, then the barrier
sponsor can operate on the frozen system, such as make snapshot.

It is implemented via new MGS/MGC plain LDLM lock on the virtual
barrier config-log. The MGS sends the barrier request to the MDTs
via glimpse callback against such barrier plain ldlm lock. The MGC
returns is local barrier status via the glimpse reply.

This patch also enhances mgs lproc interface logic.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I349249fc21e3a6394e658d7529cf87299c050f1a
Reviewed-on: https://review.whamcloud.com/24262
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>