LU-8900 snapshot: user interface for write barrier on MDT

LU-8900 snapshot: user interface for write barrier on MDT

The user can control the write barrier on MDTs via lctl commands:

Freeze barrier:
lctl barrier_freeze <fsname> [timeout (in second)]
NOTE: the default timeout value is 30 (seconds).

Thaw barrier:
lctl barrier_thaw <fsname>

Query barrier:
lctl barrier_stat <fsname>

Rescan barrier bitmap:
lctl barrier_rescan <fsname> [timeout (in second)]
NOTE: the default timeout value is 30 (seconds).

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Id953203fc3ce6ebbce9f1ae0511fbe2b3813bb9f
Reviewed-on: https://review.whamcloud.com/24265
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>